diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index e301510b..38f3e799 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -271,15 +271,19 @@ jobs:
           RUSTFLAGS: -Dwarnings --cfg getrandom_backend="unsupported"
         run: cargo build --target riscv32i-unknown-none-elf
 
-  unsupported:
-    name: Runtime error
+  wasm_override:
+    name: Web WASM override (${{ matrix.backend }})
     runs-on: ubuntu-24.04
+    strategy:
+      matrix:
+        backend: [unsupported, custom]
+    env:
+      RUSTFLAGS: -Dwarnings --cfg getrandom_backend="${{ matrix.backend }}"
     steps:
       - uses: actions/checkout@v7
       - uses: dtolnay/rust-toolchain@stable
         with:
           targets: wasm32-unknown-unknown
       - uses: Swatinem/rust-cache@v2
-      - env:
-          RUSTFLAGS: -Dwarnings --cfg getrandom_backend="unsupported"
-        run: cargo build --target wasm32-unknown-unknown
+      - run: cargo build --target wasm32-unknown-unknown
+      - run: cargo build --target wasm32-unknown-unknown --features wasm_js
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4beb2a0b..188b1ed3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Deprecation warning for `libc::INT_MAX` [#868]
+- Compilation error caused by custom backend errors when backend is
+  overridden using `getrandom_backend` configuration flag ([#876])
 
 ### Changed
 - Bumped `r-efi` dependency to v7 ([#869])
@@ -22,6 +24,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 [#869]: https://github.com/rust-random/getrandom/pull/869
 [#870]: https://github.com/rust-random/getrandom/pull/870
 [#871]: https://github.com/rust-random/getrandom/pull/871
+[#876]: https://github.com/rust-random/getrandom/pull/876
 
 ## [0.4.3] - 2026-06-17
 ### Added
diff --git a/src/backends.rs b/src/backends.rs
index d644ab62..8d8755f3 100644
--- a/src/backends.rs
+++ b/src/backends.rs
@@ -26,7 +26,10 @@ cfg_if! {
     } else if #[cfg(getrandom_backend = "efi_rng")] {
         mod efi_rng;
         pub use efi_rng::*;
-    } else if #[cfg(getrandom_backend = "windows_legacy")] {
+    } else if #[cfg(any(
+        getrandom_backend = "windows_legacy",
+        target_vendor = "win7",
+    ))] {
         mod windows_legacy;
         pub use windows_legacy::*;
     } else if #[cfg(getrandom_backend = "unsupported")] {
@@ -159,9 +162,6 @@ cfg_if! {
     } else if #[cfg(target_os = "solid_asp3")] {
         mod solid;
         pub use solid::*;
-    } else if #[cfg(all(windows, target_vendor = "win7"))] {
-        mod windows_legacy;
-        pub use windows_legacy::*;
     } else if #[cfg(windows)] {
         mod windows;
         pub use windows::*;
diff --git a/src/backends/apple_other.rs b/src/backends/apple_other.rs
index c7b51c0e..9a241774 100644
--- a/src/backends/apple_other.rs
+++ b/src/backends/apple_other.rs
@@ -14,8 +14,3 @@ pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
         Err(Error::IOS_RANDOM_GEN)
     }
 }
-
-impl Error {
-    /// Call to `CCRandomGenerateBytes` failed.
-    pub(crate) const IOS_RANDOM_GEN: Error = Self::new_internal(10);
-}
diff --git a/src/backends/rdrand.rs b/src/backends/rdrand.rs
index 6e615eb8..7c1ab6b9 100644
--- a/src/backends/rdrand.rs
+++ b/src/backends/rdrand.rs
@@ -192,10 +192,3 @@ pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     // SAFETY: After this point, we know rdrand is supported.
     unsafe { rdrand_exact(dest) }.ok_or(Error::FAILED_RDRAND)
 }
-
-impl Error {
-    /// RDRAND instruction failed due to a hardware issue.
-    pub(crate) const FAILED_RDRAND: Error = Self::new_internal(10);
-    /// RDRAND instruction unsupported on this target.
-    pub(crate) const NO_RDRAND: Error = Self::new_internal(11);
-}
diff --git a/src/backends/rndr.rs b/src/backends/rndr.rs
index 50849d50..bb8351e3 100644
--- a/src/backends/rndr.rs
+++ b/src/backends/rndr.rs
@@ -139,10 +139,3 @@ pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     // SAFETY: after this point, we know the `rand` target feature is enabled
     unsafe { rndr_fill(dest).ok_or(Error::RNDR_FAILURE) }
 }
-
-impl Error {
-    /// RNDR register read failed due to a hardware issue.
-    pub(crate) const RNDR_FAILURE: Error = Self::new_internal(10);
-    /// RNDR register is not supported on this target.
-    pub(crate) const RNDR_NOT_AVAILABLE: Error = Self::new_internal(11);
-}
diff --git a/src/backends/vxworks.rs b/src/backends/vxworks.rs
index aa07246b..c5d8b41a 100644
--- a/src/backends/vxworks.rs
+++ b/src/backends/vxworks.rs
@@ -49,8 +49,3 @@ pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     }
     Ok(())
 }
-
-impl Error {
-    /// On VxWorks, call to `randSecure` failed (random number generator is not yet initialized).
-    pub(crate) const VXWORKS_RAND_SECURE: Error = Self::new_internal(10);
-}
diff --git a/src/backends/wasm_js.rs b/src/backends/wasm_js.rs
index ee22b63f..814738cf 100644
--- a/src/backends/wasm_js.rs
+++ b/src/backends/wasm_js.rs
@@ -65,8 +65,3 @@ unsafe extern "C" {
     #[wasm_bindgen(js_namespace = ["globalThis", "crypto"], js_name = getRandomValues, catch)]
     fn get_random_values(buf: &js_sys::Uint8Array) -> Result<(), JsValue>;
 }
-
-impl Error {
-    /// The environment does not support the Web Crypto API.
-    pub(crate) const WEB_CRYPTO: Error = Self::new_internal(10);
-}
diff --git a/src/backends/windows_legacy.rs b/src/backends/windows_legacy.rs
index 0e917015..a50f8ed9 100644
--- a/src/backends/windows_legacy.rs
+++ b/src/backends/windows_legacy.rs
@@ -41,8 +41,3 @@ pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     }
     Ok(())
 }
-
-impl Error {
-    /// Call to Windows [`RtlGenRandom`](https://docs.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom) failed.
-    pub(crate) const WINDOWS_RTL_GEN_RANDOM: Error = Self::new_internal(10);
-}
diff --git a/src/error.rs b/src/error.rs
index 73a82743..cc40dc6c 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -168,6 +168,7 @@ impl Error {
             Error::UNSUPPORTED => "getrandom: this target is not supported",
             Error::ERRNO_NOT_POSITIVE => "errno: did not return a positive value",
             Error::UNEXPECTED => "unexpected situation",
+
             #[cfg(any(
                 target_os = "ios",
                 target_os = "visionos",
@@ -175,7 +176,9 @@ impl Error {
                 target_os = "tvos",
             ))]
             Error::IOS_RANDOM_GEN => "SecRandomCopyBytes: iOS Security framework failure",
-            #[cfg(all(windows, target_vendor = "win7"))]
+            #[cfg(target_os = "vxworks")]
+            Error::VXWORKS_RAND_SECURE => "randSecure: VxWorks RNG module is not initialized",
+            #[cfg(any(getrandom_backend = "windows_legacy", target_vendor = "win7",))]
             Error::WINDOWS_RTL_GEN_RANDOM => "RtlGenRandom: Windows system function failure",
             #[cfg(all(
                 feature = "wasm_js",
@@ -183,8 +186,6 @@ impl Error {
                 any(target_os = "unknown", target_os = "none")
             ))]
             Error::WEB_CRYPTO => "Web Crypto API is unavailable",
-            #[cfg(target_os = "vxworks")]
-            Error::VXWORKS_RAND_SECURE => "randSecure: VxWorks RNG module is not initialized",
 
             #[cfg(any(
                 getrandom_backend = "rdrand",
@@ -207,6 +208,57 @@ impl Error {
     }
 }
 
+// Backend-specific custom errors.
+// The errors must mirror code in `Error::internal_desc`.
+impl Error {
+    /// Call to `CCRandomGenerateBytes` failed.
+    #[cfg(any(
+        target_os = "ios",
+        target_os = "visionos",
+        target_os = "watchos",
+        target_os = "tvos",
+    ))]
+    pub(crate) const IOS_RANDOM_GEN: Error = Self::new_internal(10);
+
+    /// Call to VxWorks `randSecure` failed (random number generator is not yet initialized).
+    #[cfg(target_os = "vxworks")]
+    pub(crate) const VXWORKS_RAND_SECURE: Error = Self::new_internal(11);
+
+    /// Call to Windows `RtlGenRandom` failed.
+    #[cfg(any(getrandom_backend = "windows_legacy", target_vendor = "win7",))]
+    pub(crate) const WINDOWS_RTL_GEN_RANDOM: Error = Self::new_internal(12);
+
+    /// The environment does not support the Web Crypto API.
+    #[cfg(all(
+        feature = "wasm_js",
+        target_arch = "wasm32",
+        any(target_os = "unknown", target_os = "none")
+    ))]
+    pub(crate) const WEB_CRYPTO: Error = Self::new_internal(13);
+
+    /// RDRAND instruction failed due to a hardware issue.
+    #[cfg(any(
+        getrandom_backend = "rdrand",
+        all(target_arch = "x86_64", target_env = "sgx")
+    ))]
+    pub(crate) const FAILED_RDRAND: Error = Self::new_internal(14);
+
+    /// RDRAND instruction unsupported on this target.
+    #[cfg(any(
+        getrandom_backend = "rdrand",
+        all(target_arch = "x86_64", target_env = "sgx")
+    ))]
+    pub(crate) const NO_RDRAND: Error = Self::new_internal(15);
+
+    /// RNDR register read failed due to a hardware issue.
+    #[cfg(getrandom_backend = "rndr")]
+    pub(crate) const RNDR_FAILURE: Error = Self::new_internal(16);
+
+    /// RNDR register is not supported on this target.
+    #[cfg(getrandom_backend = "rndr")]
+    pub(crate) const RNDR_NOT_AVAILABLE: Error = Self::new_internal(17);
+}
+
 impl core::error::Error for Error {}
 
 impl fmt::Debug for Error {