diff --git a/pkgs/overlays/unstable-from-source.nix b/pkgs/overlays/unstable-from-source.nix
index 7ff3cae..cd2ac95 100644
--- a/pkgs/overlays/unstable-from-source.nix
+++ b/pkgs/overlays/unstable-from-source.nix
@@ -7,15 +7,6 @@
 
 self: super:
 
-assert self.lib.assertMsg (
-  self.lib.isFunction callPackage
-  || self.lib.isBool callPackage
-) ''
-  callPackage argument should be a function or a boolean.
-  If you want to use the callPackage from self, set it to true.
-  If you want to use a custom callPackage, set it to a function (pkgs.callPackage).
-'';
-
 let
   useUnstable = self.config.useUnstable or true;
   sanitizePlatform = platformConfig: self.lib.removeAttrs platformConfig [
@@ -49,7 +40,11 @@ let
   callPackage' = if builtins.isFunction callPackage then callPackage
     else if builtins.isBool callPackage && callPackage then self.callPackage
     else if builtins.isBool callPackage && !callPackage then throw "this should never be evaluated"
-    else throw "callPackage should be a function (callPackage) or a boolean";
+    else throw ''
+      callPackage argument should be a function or a boolean.
+      If you want to use the callPackage from self, set it to true.
+      If you want to use a custom callPackage, set it to a function (pkgs.callPackage).
+    '';
 
   callPackagesUnstablePkgs = self.lib.mapAttrsRecursiveCond (
     attrset: !(self.lib.hasAttr "override" attrset) && attrset.recurseForDerivations or false
@@ -68,6 +63,6 @@ in
 {
   "${attributeName}" = if !useUnstable then self
     # if callPackage is not false
-    else if builtins.isBool callPackage && !callPackage then callPackagesUnstablePkgs
+    else if !(builtins.isBool callPackage && !callPackage) then callPackagesUnstablePkgs
     else unstablePkgs;
 }