diff --git a/src/Backends/REFPROP/REFPROPMixtureBackend.h b/src/Backends/REFPROP/REFPROPMixtureBackend.h
index 449b1a4e..35d5a800 100644
--- a/src/Backends/REFPROP/REFPROPMixtureBackend.h
+++ b/src/Backends/REFPROP/REFPROPMixtureBackend.h
@@ -17,7 +17,7 @@ namespace CoolProp {
 
 class REFPROPMixtureBackend : public AbstractState  {
 protected:
-    int Ncomp;
+    std::size_t Ncomp;
 	bool _mole_fractions_set;
 	static bool _REFPROP_supported;
 	std::vector<double> mole_fractions, mass_fractions;