diff --git a/src/operator/nn/concat.cc b/src/operator/nn/concat.cc
index 1eeef7db5cb5..a9e744cc58d1 100644
--- a/src/operator/nn/concat.cc
+++ b/src/operator/nn/concat.cc
@@ -145,13 +145,15 @@ bool ConcatType(const nnvm::NodeAttrs& attrs,
   int dtype = -1;
 
   // checks uniformity of input
-  for (int i : *in_type) {
+  for (size_t i =0; i < in_type->size(); ++i) {
     if (dtype == -1) {
-      dtype = i;
+      dtype = in_type->at(i);
     } else {
-      CHECK(i == dtype ||
-          i == -1) <<
-          "Non-uniform data type in Concat";
+      CHECK(in_type->at(i) == dtype || in_type->at(i) == -1)
+          << "Non-uniform data type in "  << attrs.op->name
+          << ", expected data type " << mxnet::op::type_string(dtype)
+          << ", got data type " << mxnet::op::type_string(in_type->at(i))
+          << " for input " << i;
     }
   }