diff --git a/mlir/lib/Dialect/StandardOps/IR/Ops.cpp b/mlir/lib/Dialect/StandardOps/IR/Ops.cpp
--- a/mlir/lib/Dialect/StandardOps/IR/Ops.cpp
+++ b/mlir/lib/Dialect/StandardOps/IR/Ops.cpp
@@ -757,7 +757,9 @@
 
   for (unsigned i = 0, e = fnType.getNumInputs(); i != e; ++i)
     if (op.getOperand(i).getType() != fnType.getInput(i))
-      return op.emitOpError("operand type mismatch");
+      return op.emitOpError("operand ") << i << " type mismatch between " <<
+        op.getOperand(i).getType() << " and declared type " <<
+        fnType.getInput(i);
 
   if (fnType.getNumResults() != op.getNumResults())
     return op.emitOpError("incorrect number of results for callee");