diff --git a/src/uint256.h b/src/uint256.h
index 1fee31ce07..f94097f0ac 100644
--- a/src/uint256.h
+++ b/src/uint256.h
@@ -399,7 +399,7 @@ public:
 		return (unsigned char*)&pn[WIDTH];
 	}
 
-	unsigned int size()
+	unsigned int size() const
 	{
 		return sizeof(pn);
 	}