diff --git a/java/hiddenapi.go b/java/hiddenapi.go
index 2d94ac4..da2c48f 100644
--- a/java/hiddenapi.go
+++ b/java/hiddenapi.go
@@ -254,6 +254,7 @@
 	rule.Command().
 		BuiltTool("merge_csv").
 		Flag("--zip_input").
+		Flag("--key_field signature").
 		FlagWithOutput("--output=", indexCSV).
 		Inputs(classesJars)
 	rule.Build("merged-hiddenapi-index", "Merged Hidden API index")
diff --git a/java/hiddenapi_singleton.go b/java/hiddenapi_singleton.go
index 25d39f3..82e8b3f 100644
--- a/java/hiddenapi_singleton.go
+++ b/java/hiddenapi_singleton.go
@@ -424,6 +424,7 @@
 
 	rule.Command().
 		BuiltTool("merge_csv").
+		Flag("--key_field signature").
 		FlagWithOutput("--output=", outputPath).
 		Inputs(metadataCSV)
 
@@ -535,6 +536,7 @@
 	rule := android.NewRuleBuilder(pctx, ctx)
 	rule.Command().
 		BuiltTool("merge_csv").
+		Flag("--key_field signature").
 		FlagWithArg("--header=", "signature,file,startline,startcol,endline,endcol,properties").
 		FlagWithOutput("--output=", hiddenAPISingletonPaths(ctx).index).
 		Inputs(indexes)