--- a/js/src/asmjs/WasmModule.cpp
+++ b/js/src/asmjs/WasmModule.cpp
@@ -419,17 +419,17 @@ MapFile(PRFileDesc* file, PRFileInfo* in
     PRFileMap* map = PR_CreateFileMap(file, info->size, PR_PROT_READONLY);
     if (!map)
         return nullptr;
 
     // PRFileMap objects do not need to be kept alive after the memory has been
     // mapped, so unconditionally close the PRFileMap, regardless of whether
     // PR_MemMap succeeds.
     uint8_t* memory = (uint8_t*)PR_MemMap(map, 0, info->size);
-    MOZ_ALWAYS_TRUE(PR_CloseFileMap(map));
+    PR_CloseFileMap(map);
     return UniqueMapping(memory, MemUnmap(info->size));
 }
 
 bool
 wasm::CompiledModuleAssumptionsMatch(PRFileDesc* compiled, JS::BuildIdCharVector&& buildId)
 {
     PRFileInfo info;
     UniqueMapping mapping = MapFile(compiled, &info);