diff --git a/bl31/bl31_main.c b/bl31/bl31_main.c
index 861b391..ff3c53b 100644
--- a/bl31/bl31_main.c
+++ b/bl31/bl31_main.c
@@ -125,7 +125,7 @@
  ******************************************************************************/
 void bl31_set_next_image_type(uint32_t security_state)
 {
-	assert(security_state == NON_SECURE || security_state == SECURE);
+	assert(sec_state_is_valid(security_state));
 	next_image_type = security_state;
 }
 
diff --git a/bl31/context_mgmt.c b/bl31/context_mgmt.c
index 65f1213..4502e5d 100644
--- a/bl31/context_mgmt.c
+++ b/bl31/context_mgmt.c
@@ -71,7 +71,7 @@
  ******************************************************************************/
 void *cm_get_context_by_mpidr(uint64_t mpidr, uint32_t security_state)
 {
-	assert(security_state <= NON_SECURE);
+	assert(sec_state_is_valid(security_state));
 
 	return get_cpu_data_by_mpidr(mpidr, cpu_context[security_state]);
 }
@@ -82,7 +82,7 @@
  ******************************************************************************/
 void cm_set_context_by_mpidr(uint64_t mpidr, void *context, uint32_t security_state)
 {
-	assert(security_state <= NON_SECURE);
+	assert(sec_state_is_valid(security_state));
 
 	set_cpu_data_by_mpidr(mpidr, cpu_context[security_state], context);
 }
diff --git a/bl31/interrupt_mgmt.c b/bl31/interrupt_mgmt.c
index 2b0c797..e595634 100644
--- a/bl31/interrupt_mgmt.c
+++ b/bl31/interrupt_mgmt.c
@@ -107,7 +107,7 @@
 {
 	uint32_t scr_el3;
 
-	assert(security_state <= NON_SECURE);
+	assert(sec_state_is_valid(security_state));
 	scr_el3 = intr_type_descs[INTR_TYPE_NS].scr_el3[security_state];
 	scr_el3 |= intr_type_descs[INTR_TYPE_S_EL1].scr_el3[security_state];
 	scr_el3 |= intr_type_descs[INTR_TYPE_EL3].scr_el3[security_state];
diff --git a/common/bl_common.c b/common/bl_common.c
index 60b63f1..d2c60ef 100644
--- a/common/bl_common.c
+++ b/common/bl_common.c
@@ -61,12 +61,11 @@
 {
 	unsigned long scr = read_scr();
 
+	assert(sec_state_is_valid(target_security_state));
 	if (target_security_state == SECURE)
 		scr &= ~SCR_NS_BIT;
-	else if (target_security_state == NON_SECURE)
-		scr |= SCR_NS_BIT;
 	else
-		assert(0);
+		scr |= SCR_NS_BIT;
 
 	write_scr(scr);
 }
diff --git a/drivers/arm/gic/arm_gic.c b/drivers/arm/gic/arm_gic.c
index 636348b..86aaa9a 100644
--- a/drivers/arm/gic/arm_gic.c
+++ b/drivers/arm/gic/arm_gic.c
@@ -322,7 +322,7 @@
 	       type == INTR_TYPE_EL3 ||
 	       type == INTR_TYPE_NS);
 
-	assert(security_state == NON_SECURE || security_state == SECURE);
+	assert(sec_state_is_valid(security_state));
 
 	/*
 	 * We ignore the security state parameter under the assumption that
diff --git a/drivers/arm/tzc400/tzc400.c b/drivers/arm/tzc400/tzc400.c
index c1716db..715ea6c 100644
--- a/drivers/arm/tzc400/tzc400.c
+++ b/drivers/arm/tzc400/tzc400.c
@@ -103,7 +103,7 @@
 	tmp = (tzc_read_gate_keeper(base) >> GATE_KEEPER_OS_SHIFT) &
 		GATE_KEEPER_OS_MASK;
 
-	return tmp >> filter;
+	return (tmp >> filter) & GATE_KEEPER_FILTER_MASK;
 }
 
 /* This function is not MP safe. */
@@ -241,6 +241,13 @@
 	for (filter = 0; filter < controller->num_filters; filter++) {
 		state = tzc_get_gate_keeper(controller->base, filter);
 		if (state) {
+			/* The TZC filter is already configured. Changing the
+			 * programmer's view in an active system can cause
+			 * unpredictable behavior therefore panic for now rather
+			 * than try to determine whether this is safe in this
+			 * instance. See:
+			 * http://infocenter.arm.com/help/index.jsp?\
+			 * topic=/com.arm.doc.ddi0504c/CJHHECBF.html */
 			ERROR("TZC : Filter %d Gatekeeper already enabled.\n",
 				filter);
 			panic();
diff --git a/include/common/bl_common.h b/include/common/bl_common.h
index e996fd6..9945e3a 100644
--- a/include/common/bl_common.h
+++ b/include/common/bl_common.h
@@ -33,6 +33,7 @@
 
 #define SECURE		0x0
 #define NON_SECURE	0x1
+#define sec_state_is_valid(s) (((s) == SECURE) || ((s) == NON_SECURE))
 
 #define UP	1
 #define DOWN	0
diff --git a/include/drivers/arm/tzc400.h b/include/drivers/arm/tzc400.h
index b4aa3ba..03fce54 100644
--- a/include/drivers/arm/tzc400.h
+++ b/include/drivers/arm/tzc400.h
@@ -90,6 +90,7 @@
 #define GATE_KEEPER_OS_MASK	0xf
 #define GATE_KEEPER_OR_SHIFT	0
 #define GATE_KEEPER_OR_MASK	0xf
+#define GATE_KEEPER_FILTER_MASK	0x1
 
 /* Speculation is enabled by default. */
 #define SPECULATION_CTRL_WRITE_DISABLE	(1 << 1)
diff --git a/plat/fvp/aarch64/fvp_common.c b/plat/fvp/aarch64/fvp_common.c
index 3926239..d22fd55 100644
--- a/plat/fvp/aarch64/fvp_common.c
+++ b/plat/fvp/aarch64/fvp_common.c
@@ -237,7 +237,8 @@
 	counter_base_frequency = mmio_read_32(SYS_CNTCTL_BASE + CNTFID_OFF);
 
 	/* The first entry of the frequency modes table must not be 0 */
-	assert(counter_base_frequency != 0);
+	if (counter_base_frequency == 0)
+		panic();
 
 	return counter_base_frequency;
 }
diff --git a/plat/fvp/bl31_fvp_setup.c b/plat/fvp/bl31_fvp_setup.c
index ca72aa9..21fca70 100644
--- a/plat/fvp/bl31_fvp_setup.c
+++ b/plat/fvp/bl31_fvp_setup.c
@@ -92,7 +92,7 @@
 {
 #if RESET_TO_BL31
 
-	assert(type <= NON_SECURE);
+	assert(sec_state_is_valid(type));
 	SET_PARAM_HEAD(&next_image_ep_info,
 				PARAM_EP,
 				VERSION_1,
@@ -116,6 +116,8 @@
 #else
 	entry_point_info_t *next_image_info;
 
+	assert(sec_state_is_valid(type));
+
 	next_image_info = (type == NON_SECURE) ?
 		bl2_to_bl31_params->bl33_ep_info :
 		bl2_to_bl31_params->bl32_ep_info;
diff --git a/services/spd/tspd/tspd_common.c b/services/spd/tspd/tspd_common.c
index c497670..1b9609f 100644
--- a/services/spd/tspd/tspd_common.c
+++ b/services/spd/tspd/tspd_common.c
@@ -91,6 +91,7 @@
 {
 	uint64_t rc;
 
+	assert(tsp_ctx != NULL);
 	assert(tsp_ctx->c_rt_ctx == 0);
 
 	/* Apply the Secure EL1 system register context and switch to it */
@@ -117,6 +118,7 @@
  ******************************************************************************/
 void tspd_synchronous_sp_exit(tsp_context_t *tsp_ctx, uint64_t ret)
 {
+	assert(tsp_ctx != NULL);
 	/* Save the Secure EL1 system register context */
 	assert(cm_get_context(SECURE) == &tsp_ctx->cpu_ctx);
 	cm_el1_sysregs_context_save(SECURE);