diff --git a/Framework/DataObjects/test/RefAxisTest.h b/Framework/DataObjects/test/RefAxisTest.h
index 60022990c1b28171d83a316a625834dee5e25963..b2e6bcbf9c35a29c5f179b1d3fd0c1bd30e406d5 100644
--- a/Framework/DataObjects/test/RefAxisTest.h
+++ b/Framework/DataObjects/test/RefAxisTest.h
@@ -38,8 +38,8 @@ public:
 
     // Fill them
     Mantid::MantidVec a(25);
-    for (int i = 0; i < a.size(); ++i) {
-      a[i] = i + 0.1;
+    for (size_t i = 0; i < a.size(); ++i) {
+      a[i] = static_cast<double>(i) + 0.1;
     }
     for (int j = 0; j < 5; ++j) {
       m_space->mutableX(j) =