diff --git a/sparse_sys.h b/sparse_sys.h
index 8295a503..e455b0c2 100644
--- a/sparse_sys.h
+++ b/sparse_sys.h
@@ -3,6 +3,12 @@
 
 #include <stddef.h>
 
+#if defined(MATLAB_MEX_FILE) && MATLAB_MEX_FILE
+#include <mex.h>
+#undef  MAT_SIZE_T
+#define MAT_SIZE_T mwSignedIndex
+#endif
+
 #ifndef MAT_SIZE_T
 #define MAT_SIZE_T int
 #endif