diff --git a/pkgs/development/ruby-modules/gem-config/default.nix b/pkgs/development/ruby-modules/gem-config/default.nix
index 64cacc976f4..580efc2278e 100644
--- a/pkgs/development/ruby-modules/gem-config/default.nix
+++ b/pkgs/development/ruby-modules/gem-config/default.nix
@@ -115,7 +115,7 @@ in
   };
 
   rmagick = attrs: {
-    buildInputs = [ imagemagick pkgconfig ];
+    buildInputs = [ imagemagick pkgconfig which ];
   };
 
   rugged = attrs: {