diff --git a/platform/mbed_chrono.h b/platform/mbed_chrono.h
index a051588c71e..210d64b3fe2 100644
--- a/platform/mbed_chrono.h
+++ b/platform/mbed_chrono.h
@@ -19,6 +19,7 @@
 #ifndef __MBED_CHRONO_H__
 #define __MBED_CHRONO_H__
 
+#include "mbed_toolchain.h"
 #include <cstdint>
 #include <cassert>
 #include <ratio>
@@ -85,10 +86,7 @@ inline namespace chrono_literals {
 constexpr chrono::deciseconds operator "" _ds(unsigned long long x)
 {
     chrono::deciseconds::rep val = static_cast<chrono::deciseconds::rep>(x);
-    if (val < 0) {
-        assert(false);
-    }
-    assert(static_cast<unsigned long long>(val) == x);
+    assert(val >= 0 && static_cast<unsigned long long>(val) == x);
     return chrono::deciseconds(val);
 }
 
@@ -106,10 +104,7 @@ constexpr chrono::deciseconds operator "" _ds(unsigned long long x)
 constexpr chrono::centiseconds operator "" _cs(unsigned long long x)
 {
     chrono::centiseconds::rep val = static_cast<chrono::centiseconds::rep>(x);
-    if (val < 0) {
-        assert(false);
-    }
-    assert(static_cast<unsigned long long>(val) == x);
+    assert(val >= 0 && static_cast<unsigned long long>(val) == x);
     return chrono::centiseconds(val);
 }
 
diff --git a/platform/mbed_toolchain.h b/platform/mbed_toolchain.h
index a0fc28cf05e..2fa91d69e26 100644
--- a/platform/mbed_toolchain.h
+++ b/platform/mbed_toolchain.h
@@ -24,6 +24,15 @@
 #define __error_t_defined 1
 #endif
 
+/* Work around ARM Compiler 6 bug - assert does not work in constexpr
+ * functions unless you stop it from using its __promise built-in.
+ */
+#ifdef __ARMCC_VERSION
+#ifndef __DO_NOT_LINK_PROMISE_WITH_ASSERT
+#define __DO_NOT_LINK_PROMISE_WITH_ASSERT
+#endif
+#endif
+
 // Warning for unsupported compilers
 #if !defined(__GNUC__)   /* GCC        */ \
  && !defined(__clang__)  /* LLVM/Clang */ \