diff --git a/src/Base/Unit.cpp b/src/Base/Unit.cpp
index 569a40ebbc..e63b321eb9 100644
--- a/src/Base/Unit.cpp
+++ b/src/Base/Unit.cpp
@@ -127,7 +127,7 @@ Unit::Unit(const QString& expr)
     }
 }
 
-Unit Unit::pow(char exp) const
+Unit Unit::pow(signed char exp) const
 {
     checkRange("pow()",
                (int32_t)Sig.Length * (int32_t)exp,
diff --git a/src/Base/Unit.h b/src/Base/Unit.h
index 7b0b335c0c..115d14eeab 100644
--- a/src/Base/Unit.h
+++ b/src/Base/Unit.h
@@ -79,7 +79,7 @@ public:
     bool operator ==(const Unit&) const;
     bool operator !=(const Unit&that) const {return !(*this == that);}
     Unit& operator =(const Unit&);
-    Unit pow(char exp)const;
+    Unit pow(signed char exp)const;
     //@}
     /// get the unit signature
     const UnitSignature & getSignature(void)const {return Sig;}