diff --git a/ndk_compat/cpu-features.c b/ndk_compat/cpu-features.c
index 27ff7bbd..c7d11be9 100644
--- a/ndk_compat/cpu-features.c
+++ b/ndk_compat/cpu-features.c
@@ -27,7 +27,11 @@ static uint32_t g_cpuIdArm;
 #endif
 
 static void set_cpu_mask_bit(uint32_t index, uint32_t* cpu_mask) {
-  *cpu_mask |= 1UL << index;
+  // cpu_mask is a 32-bit mask; a larger index would shift past its width
+  // (shifting by >= the operand width is undefined behavior). The range path
+  // in parse_cpu_mask bounds indices the same way.
+  if (index >= 32) return;
+  *cpu_mask |= (uint32_t)1 << index;
 }
 
 // Examples of valid inputs: "31", "4-31"