diff --git a/mlir/lib/TableGen/AttrOrTypeDef.cpp b/mlir/lib/TableGen/AttrOrTypeDef.cpp
--- a/mlir/lib/TableGen/AttrOrTypeDef.cpp
+++ b/mlir/lib/TableGen/AttrOrTypeDef.cpp
@@ -81,7 +81,13 @@
                     "'assemblyFormat' or 'hasCustomAssemblyFormat' can only be "
                     "used when 'mnemonic' is set");
   }
-  // Assembly format requires accessors to be generated.
+  // Assembly format parser requires default builders to be generated.
+  if (hasDeclarativeFormat && skipDefaultBuilders()) {
+    PrintFatalError(
+        getLoc(),
+        "'assemblyFormat' requires 'skipDefaultBuilders' to be false");
+  }
+  // Assembly format printer requires accessors to be generated.
   if (hasDeclarativeFormat && !genAccessors()) {
     PrintFatalError(getLoc(),
                     "'assemblyFormat' requires 'genAccessors' to be true");
diff --git a/mlir/lib/TableGen/Builder.cpp b/mlir/lib/TableGen/Builder.cpp
--- a/mlir/lib/TableGen/Builder.cpp
+++ b/mlir/lib/TableGen/Builder.cpp
@@ -21,8 +21,10 @@
 StringRef Builder::Parameter::getCppType() const {
   if (const auto *stringInit = dyn_cast<llvm::StringInit>(def))
     return stringInit->getValue();
-  const llvm::Record *record = cast<llvm::DefInit>(def)->getDef();
-  return record->getValueAsString("type");
+  if (const llvm::Record *record = cast<llvm::DefInit>(def)->getDef())
+    return record->getValueAsString("type");
+  llvm::PrintFatalError("Builder DAG arguments must be either strings or defs "
+                        "which inherit from CArg\n");
 }
 
 /// Return an optional string containing the default value to use for this