diff --git a/js/public/CharacterEncoding.h b/js/public/CharacterEncoding.h
index af040ffbc4a3..fe39a415c77f 100644
--- a/js/public/CharacterEncoding.h
+++ b/js/public/CharacterEncoding.h
@@ -323,6 +323,13 @@ UTF8CharsToNewLatin1CharsZ(JSContext* cx, const UTF8Chars utf8, size_t* outlen);
 extern Latin1CharsZ
 LossyUTF8CharsToNewLatin1CharsZ(JSContext* cx, const UTF8Chars utf8, size_t* outlen);
 
+/*
+ * Returns true if all characters in the given null-terminated string are
+ * ASCII, i.e. < 0x80, false otherwise.
+ */
+extern bool
+StringIsASCII(const char* s);
+
 } // namespace JS
 
 inline void JS_free(JS::Latin1CharsZ& ptr) { js_free((void*)ptr.get()); }
diff --git a/js/src/jscntxt.cpp b/js/src/jscntxt.cpp
index 9b064aff76ad..7bb4d8b36565 100644
--- a/js/src/jscntxt.cpp
+++ b/js/src/jscntxt.cpp
@@ -317,17 +317,6 @@ checkReportFlags(JSContext* cx, unsigned* flags)
     return false;
 }
 
-#ifdef DEBUG
-static void
-AssertIsASCII(const char* s)
-{
-    while (*s) {
-        MOZ_ASSERT((*s & 0x80) == 0);
-        s++;
-    }
-}
-#endif
-
 bool
 js::ReportErrorVA(JSContext* cx, unsigned flags, const char* format,
                   ErrorArgumentsType argumentsType, va_list ap)
@@ -348,10 +337,7 @@ js::ReportErrorVA(JSContext* cx, unsigned flags, const char* format,
     }
     messagelen = strlen(message);
 
-#ifdef DEBUG
-    if (argumentsType == ArgumentsAreASCII)
-        AssertIsASCII(message);
-#endif
+    MOZ_ASSERT_IF(argumentsType == ArgumentsAreASCII, JS::StringIsASCII(message));
 
     report.flags = flags;
     report.errorNumber = JSMSG_USER_DEFINED_ERROR;
@@ -570,10 +556,7 @@ class MOZ_RAII AutoMessageArgs
                 const char* charArg = va_arg(ap, char*);
                 size_t charArgLength = strlen(charArg);
 
-#ifdef DEBUG
-                if (typeArg == ArgumentsAreASCII)
-                    AssertIsASCII(charArg);
-#endif
+                MOZ_ASSERT_IF(typeArg == ArgumentsAreASCII, JS::StringIsASCII(charArg));
 
                 args_[i] = InflateString(cx, charArg, &charArgLength);
                 if (!args_[i])
@@ -631,10 +614,7 @@ js::ExpandErrorArgumentsVA(ExclusiveContext* cx, JSErrorCallback callback,
     if (efs) {
         reportp->exnType = efs->exnType;
 
-#ifdef DEBUG
-        if (argumentsType == ArgumentsAreASCII)
-            AssertIsASCII(efs->format);
-#endif
+        MOZ_ASSERT_IF(argumentsType == ArgumentsAreASCII, JS::StringIsASCII(efs->format));
 
         uint16_t argCount = efs->argCount;
         MOZ_RELEASE_ASSERT(argCount <= JS::MaxNumErrorArguments);
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index b3fedad81f31..24a95b34bce1 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -3790,7 +3790,7 @@ Compile(JSContext* cx, unsigned argc, Value* vp)
     }
     if (!args[0].isString()) {
         const char* typeName = InformalValueTypeName(args[0]);
-        JS_ReportError(cx, "expected string to compile, got %s", typeName);
+        JS_ReportErrorASCII(cx, "expected string to compile, got %s", typeName);
         return false;
     }
 
@@ -3827,7 +3827,7 @@ ParseModule(JSContext* cx, unsigned argc, Value* vp)
 
     if (!args[0].isString()) {
         const char* typeName = InformalValueTypeName(args[0]);
-        JS_ReportError(cx, "expected string to compile, got %s", typeName);
+        JS_ReportErrorASCII(cx, "expected string to compile, got %s", typeName);
         return false;
     }
 
@@ -3840,7 +3840,7 @@ ParseModule(JSContext* cx, unsigned argc, Value* vp)
     if (args.length() > 1) {
         if (!args[1].isString()) {
             const char* typeName = InformalValueTypeName(args[1]);
-            JS_ReportError(cx, "expected filename string, got %s", typeName);
+            JS_ReportErrorASCII(cx, "expected filename string, got %s", typeName);
             return false;
         }
 
@@ -3882,7 +3882,7 @@ SetModuleResolveHook(JSContext* cx, unsigned argc, Value* vp)
 
     if (!args[0].isObject() || !args[0].toObject().is<JSFunction>()) {
         const char* typeName = InformalValueTypeName(args[0]);
-        JS_ReportError(cx, "expected hook function, got %s", typeName);
+        JS_ReportErrorASCII(cx, "expected hook function, got %s", typeName);
         return false;
     }
 
@@ -3915,7 +3915,7 @@ Parse(JSContext* cx, unsigned argc, Value* vp)
     }
     if (!args[0].isString()) {
         const char* typeName = InformalValueTypeName(args[0]);
-        JS_ReportError(cx, "expected string to parse, got %s", typeName);
+        JS_ReportErrorASCII(cx, "expected string to parse, got %s", typeName);
         return false;
     }
 
@@ -3966,7 +3966,7 @@ SyntaxParse(JSContext* cx, unsigned argc, Value* vp)
     }
     if (!args[0].isString()) {
         const char* typeName = InformalValueTypeName(args[0]);
-        JS_ReportError(cx, "expected string to parse, got %s", typeName);
+        JS_ReportErrorASCII(cx, "expected string to parse, got %s", typeName);
         return false;
     }
 
@@ -4031,7 +4031,7 @@ OffThreadCompileScript(JSContext* cx, unsigned argc, Value* vp)
     }
     if (!args[0].isString()) {
         const char* typeName = InformalValueTypeName(args[0]);
-        JS_ReportError(cx, "expected string to parse, got %s", typeName);
+        JS_ReportErrorASCII(cx, "expected string to parse, got %s", typeName);
         return false;
     }
 
diff --git a/js/src/vm/CharacterEncoding.cpp b/js/src/vm/CharacterEncoding.cpp
index 74fe3cc06384..1d28511bae23 100644
--- a/js/src/vm/CharacterEncoding.cpp
+++ b/js/src/vm/CharacterEncoding.cpp
@@ -518,3 +518,14 @@ JS::ConstUTF8CharsZ::validate(size_t aLength)
         /* smallestEncoding = */ nullptr);
 }
 #endif
+
+bool
+JS::StringIsASCII(const char* s)
+{
+    while (*s) {
+        if (*s & 0x80)
+            return false;
+        s++;
+    }
+    return true;
+}
diff --git a/js/src/vm/ObjectGroup.cpp b/js/src/vm/ObjectGroup.cpp
index 701817bd28d0..9a9831f552f8 100644
--- a/js/src/vm/ObjectGroup.cpp
+++ b/js/src/vm/ObjectGroup.cpp
@@ -13,6 +13,7 @@
 #include "gc/Policy.h"
 #include "gc/StoreBuffer.h"
 #include "gc/Zone.h"
+#include "js/CharacterEncoding.h"
 #include "vm/ArrayObject.h"
 #include "vm/TaggedProto.h"
 #include "vm/UnboxedObject.h"
@@ -37,6 +38,7 @@ ObjectGroup::ObjectGroup(const Class* clasp, TaggedProto proto, JSCompartment* c
 
     /* Windows may not appear on prototype chains. */
     MOZ_ASSERT_IF(proto.isObject(), !IsWindow(proto.toObject()));
+    MOZ_ASSERT(JS::StringIsASCII(clasp->name));
 
     this->clasp_ = clasp;
     this->proto_ = proto;
diff --git a/js/src/vm/ObjectGroup.h b/js/src/vm/ObjectGroup.h
index 4bc3e69cd74b..4e24de9f14b8 100644
--- a/js/src/vm/ObjectGroup.h
+++ b/js/src/vm/ObjectGroup.h
@@ -12,6 +12,7 @@
 
 #include "ds/IdValuePair.h"
 #include "gc/Barrier.h"
+#include "js/CharacterEncoding.h"
 #include "js/GCHashTable.h"
 #include "vm/TaggedProto.h"
 #include "vm/TypeInference.h"
@@ -102,6 +103,7 @@ class ObjectGroup : public gc::TenuredCell
     }
 
     void setClasp(const Class* clasp) {
+        MOZ_ASSERT(JS::StringIsASCII(clasp->name));
         clasp_ = clasp;
     }