diff --git a/operator-assign.go b/operator-assign.go
index 86bcc58..6e077c1 100644
--- a/operator-assign.go
+++ b/operator-assign.go
@@ -76,14 +76,18 @@ func evalAssign(ctx ExprContext, opTerm *term) (v any, err error) {
 
 	if v, err = rightChild.compute(ctx); err == nil {
 		if functor, ok := v.(Functor); ok {
-			if info := functor.GetFunc(); info != nil {
-				ctx.RegisterFunc(leftTerm.source(), info.Functor(), info.ReturnType(), info.Params())
-			} else if funcDef, ok := functor.(*exprFunctor); ok {
-				paramSpecs := ForAll(funcDef.params, func(p ExprFuncParam) ExprFuncParam { return p })
+			if leftSym == SymVariable {
+				if info := functor.GetFunc(); info != nil {
+					ctx.RegisterFunc(leftTerm.source(), info.Functor(), info.ReturnType(), info.Params())
+				} else if funcDef, ok := functor.(*exprFunctor); ok {
+					paramSpecs := ForAll(funcDef.params, func(p ExprFuncParam) ExprFuncParam { return p })
 
-				ctx.RegisterFunc(leftTerm.source(), functor, TypeAny, paramSpecs)
+					ctx.RegisterFunc(leftTerm.source(), functor, TypeAny, paramSpecs)
+				} else {
+					err = opTerm.Errorf("unknown function %s()", rightChild.source())
+				}
 			} else {
-				err = opTerm.Errorf("unknown function %s()", rightChild.source())
+				err = assignValue(ctx, leftTerm, v)
 			}
 		} else {
 			err = assignValue(ctx, leftTerm, v)