diff --git a/src/tests/common-files/gnuplot_i.h b/src/tests/common-files/gnuplot_i.h
index 615e9084e..a3bdd1c92 100644
--- a/src/tests/common-files/gnuplot_i.h
+++ b/src/tests/common-files/gnuplot_i.h
@@ -1596,9 +1596,9 @@ Gnuplot& Gnuplot::plot_image(const unsigned char * ucPicBuf,
     // write the data to file
     //
     int iIndex = 0;
-    for(int iRow = 0; iRow < iHeight; iRow++)
+    for(unsigned int iRow = 0; iRow < iHeight; iRow++)
         {
-            for(int iColumn = 0; iColumn < iWidth; iColumn++)
+            for(unsigned int iColumn = 0; iColumn < iWidth; iColumn++)
                 {
                     tmp << iColumn << " " << iRow  << " "
                             << static_cast<float>(ucPicBuf[iIndex++]) << std::endl;