diff --git a/Code/Mantid/Framework/DataHandling/src/LoadFITS.cpp b/Code/Mantid/Framework/DataHandling/src/LoadFITS.cpp
index 7d5e0789118c4bcd56e761619c0ef2f2cffc62b2..04fed7e0ae65cc2bc711a4594691824c4b96ac04 100644
--- a/Code/Mantid/Framework/DataHandling/src/LoadFITS.cpp
+++ b/Code/Mantid/Framework/DataHandling/src/LoadFITS.cpp
@@ -377,7 +377,7 @@ namespace DataHandling
 
 		// Now load chunk into workspace 
 		PARALLEL_FOR1(workspace)
-		for (size_t wi = 0; wi < spectraCount; ++wi)
+		for (int64_t wi = 0; wi < static_cast<int64_t>(spectraCount); ++wi)
 		{
 			workspace->setX(wi, x);
 			MantidVec *currY = &workspace->dataY(wi);