diff --git a/mlir/lib/Parser/AttributeParser.cpp b/mlir/lib/Parser/AttributeParser.cpp
--- a/mlir/lib/Parser/AttributeParser.cpp
+++ b/mlir/lib/Parser/AttributeParser.cpp
@@ -840,7 +840,10 @@
   if (getToken().isNot(Token::string))
     return (emitError("expected dialect namespace"), nullptr);
 
-  std::string name = getToken().getStringValue();
+  StringRef name = getToken().getStringValue();
+  if (!Dialect::isValidNamespace(name))
+    return (emitError() << "invalid dialect namespace '" << name << "'",
+            nullptr);
   consumeToken(Token::string);
 
   if (parseToken(Token::comma, "expected ','"))
diff --git a/mlir/test/IR/attribute.mlir b/mlir/test/IR/attribute.mlir
--- a/mlir/test/IR/attribute.mlir
+++ b/mlir/test/IR/attribute.mlir
@@ -645,3 +645,7 @@
   return
 }
 
+// -----
+
+// expected-error @+1 {{invalid dialect namespace 'string with space'}}
+#invalid_dialect = opaque<"string with space", "0xDEADBEEF"> : i32