diff --git a/lib/bash/std/lib_std.sh b/lib/bash/std/lib_std.sh
index 15b0fc6..d71af4d 100644
--- a/lib/bash/std/lib_std.sh
+++ b/lib/bash/std/lib_std.sh
@@ -3618,9 +3618,14 @@ __base_bash_libs_std_validate_array_kind__() {
     (($# >= 3)) || return 1
     local __base_bash_libs_std_validate_array_operation="$1" __base_bash_libs_std_validate_array_kind="$2"
     local __base_bash_libs_std_validate_array_name __base_bash_libs_std_validate_array_label=indexed
+    local -i __base_bash_libs_std_validate_array_kind_code=0
     shift 2
 
-    [[ "$__base_bash_libs_std_validate_array_kind" == "A" ]] &&
+    # Compare the marker byte numerically so caller `nocasematch` cannot make
+    # Bash classify the indexed marker `a` as the associative marker `A`.
+    printf -v __base_bash_libs_std_validate_array_kind_code '%d' \
+        "'$__base_bash_libs_std_validate_array_kind"
+    ((__base_bash_libs_std_validate_array_kind_code == 65)) &&
         __base_bash_libs_std_validate_array_label=associative
     __base_bash_libs_std_validate_variable_names__ \
         "$__base_bash_libs_std_validate_array_operation" "$@" || return 1
diff --git a/lib/bash/std/tests/lib_std.bats b/lib/bash/std/tests/lib_std.bats
index 518c6e9..b3da033 100644
--- a/lib/bash/std/tests/lib_std.bats
+++ b/lib/bash/std/tests/lib_std.bats
@@ -4575,6 +4575,23 @@ EOF
     shopt -q nocasematch
 }
 
+@test "array-kind validation keeps indexed diagnostics case-sensitive with nocasematch" {
+    local -A associative_values=([alpha]=one)
+    local stderr_file="$TEST_TMPDIR/array-kind-validation.err"
+    local status
+
+    shopt -s nocasematch
+    if __base_bash_libs_std_validate_array_kind__ base_std_test a associative_values 2>"$stderr_file"; then
+        status=0
+    else
+        status=$?
+    fi
+    shopt -u nocasematch
+
+    [ "$status" -eq 1 ]
+    [[ "$(cat "$stderr_file")" == *"must be a caller-declared indexed array."* ]]
+}
+
 @test "base_std_assert_associative_array accepts declared associative arrays" {
     local -A values=([alpha]="one")
 
diff --git a/lib/bash/str/tests/lib_str.bats b/lib/bash/str/tests/lib_str.bats
index d7cb99e..0f7297b 100644
--- a/lib/bash/str/tests/lib_str.bats
+++ b/lib/bash/str/tests/lib_str.bats
@@ -195,7 +195,7 @@ create_script() {
 
     [ "$rc" -eq 2 ]
     [ "${fields[keep]}" = sentinel ]
-    [[ "$(<"$stderr_file")" == *"attributes incompatible with the indexed-array output contract"* ]]
+    [[ "$(<"$stderr_file")" == *"must be a caller-declared indexed array"* ]]
 }
 
 @test "readonly string outputs cannot collide with argument-count decimal locals" {