diff --git a/helper/diff/resource_builder.go b/helper/diff/resource_builder.go
index 00a172f43..53f1a6839 100644
--- a/helper/diff/resource_builder.go
+++ b/helper/diff/resource_builder.go
@@ -96,7 +96,7 @@ func (b *ResourceBuilder) Diff(
 
 				// If we have a pre-processor for this, run it.
 				if pp, ok := b.PreProcess[k]; ok {
-					v = pp(k)
+					v = pp(v)
 				}
 			}
 
diff --git a/helper/diff/resource_builder_test.go b/helper/diff/resource_builder_test.go
index dac74ff85..8f2a99c86 100644
--- a/helper/diff/resource_builder_test.go
+++ b/helper/diff/resource_builder_test.go
@@ -206,8 +206,8 @@ func TestResourceBuilder_preProcess(t *testing.T) {
 		},
 
 		PreProcess: map[string]PreProcessFunc{
-			"foo": func(string) string {
-				return "bar"
+			"foo": func(v string) string {
+				return "bar" + v
 			},
 		},
 	}
@@ -413,7 +413,7 @@ const testRBNewDiff = `UPDATE
 `
 
 const testRBPreProcessDiff = `CREATE
-  IN  foo: "" => "bar" (forces new resource)
+  IN  foo: "" => "barfoo" (forces new resource)
 `
 
 const testRBPreProcessUnknownDiff = `CREATE