diff --git a/library/constant_time_impl.h b/library/constant_time_impl.h
index 971388c19..54bd2b0ed 100644
--- a/library/constant_time_impl.h
+++ b/library/constant_time_impl.h
@@ -183,8 +183,14 @@ static inline mbedtls_ct_condition_t mbedtls_ct_bool(mbedtls_ct_uint_t x)
 #pragma warning( push )
 #pragma warning( disable : 4146 )
 #endif
-    return (mbedtls_ct_condition_t) (((mbedtls_ct_int_t) ((-xo) | -(xo >> 1))) >>
-                                     (MBEDTLS_CT_SIZE - 1));
+    // y is negative (i.e., top bit set) iff x is non-zero
+    mbedtls_ct_int_t y = (-xo) | -(xo >> 1);
+
+    // extract only the sign bit of y so that y == 1 (if x is non-zero) or 0 (if x is zero)
+    y = (((mbedtls_ct_uint_t) y) >> (MBEDTLS_CT_SIZE - 1));
+
+    // -y has all bits set (if x is non-zero), or all bits clear (if x is zero)
+    return (mbedtls_ct_condition_t) (-y);
 #if defined(_MSC_VER)
 #pragma warning( pop )
 #endif