diff --git a/libsrc/geom2d/csg2d.cpp b/libsrc/geom2d/csg2d.cpp
index d1871166..af05475a 100644
--- a/libsrc/geom2d/csg2d.cpp
+++ b/libsrc/geom2d/csg2d.cpp
@@ -1380,19 +1380,19 @@ Solid2d Solid2d :: operator-(const Solid2d & other_) const
   return res;
 }
 
-Solid2d Solid2d :: operator+=(const Solid2d & other)
+Solid2d & Solid2d :: operator+=(const Solid2d & other)
 {
   *this = *this + other;
   return *this;
 }
 
-Solid2d Solid2d :: operator*=(const Solid2d & other)
+Solid2d & Solid2d :: operator*=(const Solid2d & other)
 {
   *this = *this * other;
   return *this;
 }
 
-Solid2d Solid2d :: operator-=(const Solid2d & other)
+Solid2d & Solid2d :: operator-=(const Solid2d & other)
 {
   *this = *this - other;
   return *this;
diff --git a/libsrc/geom2d/csg2d.hpp b/libsrc/geom2d/csg2d.hpp
index a7a92b12..42054bd5 100644
--- a/libsrc/geom2d/csg2d.hpp
+++ b/libsrc/geom2d/csg2d.hpp
@@ -575,9 +575,9 @@ struct Solid2d
   Solid2d operator-(const Solid2d & other) const;
 
   Solid2d& operator=(const Solid2d & other) = default;
-  Solid2d operator+=(const Solid2d & other);
-  Solid2d operator*=(const Solid2d & other);
-  Solid2d operator-=(const Solid2d & other);
+  Solid2d& operator+=(const Solid2d & other);
+  Solid2d& operator*=(const Solid2d & other);
+  Solid2d& operator-=(const Solid2d & other);
 
   void Append( const Loop & poly )
   {