diff --git a/class.go b/class.go
index 8aaf0a1..2089444 100644
--- a/class.go
+++ b/class.go
@@ -115,12 +115,12 @@ func (mrb *State) vmDefineClass(outer Value, super Value, id Symbol) (RClass, er
 		if ClassP(super) {
 			superClass = super.(RClass)
 		} else {
-			panic("super is not a class")
+			mrb.Raisef(nil, "superclass must be a Class (%T given)", super)
 		}
 	}
 
 	if !ClassPointerP(outer) {
-		panic("outer is not a class or module")
+		mrb.Raisef(nil, "outer is not a class or module")
 	}
 	outerModule := outer.(RClass)
 
diff --git a/vm.go b/vm.go
index b760c75..4e18546 100644
--- a/vm.go
+++ b/vm.go
@@ -346,7 +346,7 @@ func (mrb *State) VmExec(proc RProc, code *insn.Sequence) (ret Value, err error)
 
 			recv, ok := ctx.Get(int(a)).(RClass)
 			if !ok {
-				panic(fmt.Errorf("not a class: %v", ctx.Get(int(a))))
+				mrb.Raisef(nil, "not a class: %v", ctx.Get(int(a)))
 			}
 
 			nirep := rep.Representation(b)