diff --git a/lib/libthr/arch/powerpc/powerpc/_umtx_op_err.S b/lib/libthr/arch/powerpc/powerpc/_umtx_op_err.S
index a902a7360d7..a0491e80a34 100644
--- a/lib/libthr/arch/powerpc/powerpc/_umtx_op_err.S
+++ b/lib/libthr/arch/powerpc/powerpc/_umtx_op_err.S
@@ -35,6 +35,11 @@
 ENTRY(_umtx_op_err)
 	li	%r0, SYS__umtx_op
 	sc
+	/* If summary overflow is set, r3 already contains the error. */
+	bsolr
+	/* Overwrite syscall aux return values. */
+	li	%r3, 0
+	li	%r4, 0
 	blr
 END(_umtx_op_err)