diff --git a/tb/sw/src/mstStartup.S b/tb/sw/src/mstStartup.S
index cc03fb2..7fe8c9f 100644
--- a/tb/sw/src/mstStartup.S
+++ b/tb/sw/src/mstStartup.S
@@ -10,6 +10,7 @@
 
 _prog_start:
 	li x15, 0
+	li sp, 0
 	# picorv32_maskirq_insn(zero, zero)
 	call main