diff --git a/lib/psci/psci_on.c b/lib/psci/psci_on.c
index c70b377..6c6b23c 100644
--- a/lib/psci/psci_on.c
+++ b/lib/psci/psci_on.c
@@ -62,12 +62,17 @@
 	int rc;
 	aff_info_state_t target_aff_state;
 	int ret = plat_core_pos_by_mpidr(target_cpu);
-	unsigned int target_idx = (unsigned int)ret;
+	unsigned int target_idx;
 
 	/* Calling function must supply valid input arguments */
-	assert(ret >= 0);
 	assert(ep != NULL);
 
+	if ((ret < 0) || (ret >= (int)PLATFORM_CORE_COUNT)) {
+		ERROR("Unexpected core index.\n");
+		panic();
+	}
+
+	target_idx = (unsigned int)ret;
 
 	/*
 	 * This function must only be called on platforms where the