diff --git a/terraform/eval_refresh.go b/terraform/eval_refresh.go
index 1d4e87af0..a13ef5de7 100644
--- a/terraform/eval_refresh.go
+++ b/terraform/eval_refresh.go
@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"log"
 
+	"github.com/zclconf/go-cty/cty"
+
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/providers"
 	"github.com/hashicorp/terraform/states"
@@ -62,6 +64,13 @@ func (n *EvalRefresh) Eval(ctx EvalContext) (interface{}, error) {
 		return nil, diags.Err()
 	}
 
+	if resp.NewState == cty.NilVal {
+		// This ought not to happen in real cases since it's not possible to
+		// send NilVal over the plugin RPC channel, but it can come up in
+		// tests due to sloppy mocking.
+		panic("new state is cty.NilVal")
+	}
+
 	for _, err := range schema.ImpliedType().TestConformance(resp.NewState.Type()) {
 		diags = diags.Append(tfdiags.Sourceless(
 			tfdiags.Error,