diff --git a/services/std_svc/psci/psci_on.c b/services/std_svc/psci/psci_on.c
index 200e622..9d2295e 100644
--- a/services/std_svc/psci/psci_on.c
+++ b/services/std_svc/psci/psci_on.c
@@ -74,6 +74,10 @@
 	unsigned int target_idx = plat_core_pos_by_mpidr(target_cpu);
 	aff_info_state_t target_aff_state;
 
+	/* Calling function must supply valid input arguments */
+	assert((int) target_idx >= 0);
+	assert(ep != NULL);
+
 	/*
 	 * This function must only be called on platforms where the
 	 * CPU_ON platform hooks have been implemented.