diff --git a/bl31/aarch64/runtime_exceptions.S b/bl31/aarch64/runtime_exceptions.S
index 57c065c..60be932 100644
--- a/bl31/aarch64/runtime_exceptions.S
+++ b/bl31/aarch64/runtime_exceptions.S
@@ -390,12 +390,12 @@
 	 * content). Either way, we aren't leaking any secure information
 	 * through them.
 	 */
-	mov	w0, #SMC_UNK
+	mov	x0, #SMC_UNK
 	b	restore_gp_registers_callee_eret
 
 smc_prohibited:
 	ldr	x30, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_LR]
-	mov	w0, #SMC_UNK
+	mov	x0, #SMC_UNK
 	eret
 
 rt_svc_fw_critical_error:
diff --git a/include/lib/smcc.h b/include/lib/smcc.h
index a273b3a..0a2d927 100644
--- a/include/lib/smcc.h
+++ b/include/lib/smcc.h
@@ -35,13 +35,13 @@
 #define SMC_64				U(1)
 #define SMC_32				U(0)
 #define SMC_OK				U(0)
-#define SMC_UNK				U(0xffffffff)
+#define SMC_UNK				-1
 #define SMC_TYPE_FAST			ULL(1)
 #if !ERROR_DEPRECATED
 #define SMC_TYPE_STD			ULL(0)
 #endif
 #define SMC_TYPE_YIELD			U(0)
-#define SMC_PREEMPTED		U(0xfffffffe)
+#define SMC_PREEMPTED			-2
 /*******************************************************************************
  * Owning entity number definitions inside the function id as per the SMC
  * calling convention
@@ -99,7 +99,7 @@
  */
 #define DEFINE_SVC_UUID(_name, _tl, _tm, _th, _cl, _ch, \
 		_n0, _n1, _n2, _n3, _n4, _n5) \
-	CASSERT(_tl != SMC_UNK, invalid_svc_uuid);\
+	CASSERT((uint32_t)(_tl) != (uint32_t) SMC_UNK, invalid_svc_uuid);\
 	static const uuid_t _name = { \
 		_tl, _tm, _th, _cl, _ch, \
 		{ _n0, _n1, _n2, _n3, _n4, _n5 } \