diff --git a/src/units.hh b/src/units.hh
index a99957f9..99e2db45 100644
--- a/src/units.hh
+++ b/src/units.hh
@@ -53,12 +53,12 @@ public:
     RealType operator--(int)
     { RealType backup(static_cast<RealType&>(*this)); --m_value; return backup; }
 
-    constexpr RealType operator-() { return RealType(-m_value); }
+    constexpr RealType operator-() const { return RealType(-m_value); }
 
     constexpr RealType operator%(RealType other) const
     { return RealType(m_value % other.m_value); }
 
-    constexpr RealType operator%=(RealType other)
+    RealType& operator%=(RealType other)
     { m_value %= other.m_value; return static_cast<RealType&>(*this); }
 
     constexpr bool operator==(RealType other) const