diff --git a/src/conf.h b/src/conf.h
index b4c27943..98c49441 100644
--- a/src/conf.h
+++ b/src/conf.h
@@ -78,8 +78,9 @@ ACC_COMPILE_TIME_ASSERT_HEADER(-1 == ~0) // two's complement - see http://wg21.l
 ACC_COMPILE_TIME_ASSERT_HEADER(0u-1 == ~0u) // two's complement - see http://wg21.link/P0907R4
 ACC_COMPILE_TIME_ASSERT_HEADER((1u << 31) << 1 == 0)
 ACC_COMPILE_TIME_ASSERT_HEADER(((int)(1u << 31)) >> 31 == -1) // arithmetic right shift
+ACC_COMPILE_TIME_ASSERT_HEADER((-1) >> 31 == -1) // arithmetic right shift
 ACC_COMPILE_TIME_ASSERT_HEADER(CHAR_MAX == 255) // -funsigned-char
-ACC_COMPILE_TIME_ASSERT_HEADER((char)(-1) > 0) // -funsigned-char
+ACC_COMPILE_TIME_ASSERT_HEADER((char)(-1) == 255) // -funsigned-char
 
 // enable/disable some warnings
     // Some non-GLIBC toolchains do not use 'nullptr' everywhere when C++:
@@ -88,6 +89,8 @@ ACC_COMPILE_TIME_ASSERT_HEADER((char)(-1) > 0) // -funsigned-char
     //   51:32: error: zero as null pointer constant
 #if (ACC_CC_GNUC >= 0x040700) && defined(__GLIBC__)
 #  pragma GCC diagnostic error "-Wzero-as-null-pointer-constant"
+#elif (ACC_CC_CLANG >= 0x050000ul)
+#  pragma clang diagnostic error "-Wzero-as-null-pointer-constant"
 #endif
 
 #if (ACC_CC_MSC)
@@ -325,7 +328,7 @@ inline void NO_fprintf(FILE *, const char *, ...) {}
      typedef a acc_tmp_a_t; typedef b acc_tmp_b_t; \
      __packed_struct(acc_tmp_t) acc_tmp_b_t x; acc_tmp_a_t y; acc_tmp_b_t z; __packed_struct_end() \
      COMPILE_TIME_ASSERT(sizeof(struct acc_tmp_t) == 2*sizeof(b)+sizeof(a)) \
-     COMPILE_TIME_ASSERT(sizeof(((acc_tmp_t*)0)->x)+sizeof(((acc_tmp_t*)0)->y)+sizeof(((acc_tmp_t*)0)->z) == 2*sizeof(b)+sizeof(a)) \
+     COMPILE_TIME_ASSERT(sizeof(((acc_tmp_t*)nullptr)->x)+sizeof(((acc_tmp_t*)nullptr)->y)+sizeof(((acc_tmp_t*)nullptr)->z) == 2*sizeof(b)+sizeof(a)) \
    }
 #if defined(__acc_alignof)
 #  define __COMPILE_TIME_ASSERT_ALIGNOF(a,b) \
diff --git a/src/miniacc.h b/src/miniacc.h
index d598bfa3..7c337487 100644
--- a/src/miniacc.h
+++ b/src/miniacc.h
@@ -4822,7 +4822,7 @@ typedef unsigned short wchar_t;
 #endif
 #endif
 #ifndef offsetof
-#define offsetof(s,m)   ((size_t)((ptrdiff_t)&(((s*)0)->m)))
+#define offsetof(s,m)   ((size_t)((ptrdiff_t)&(((s*)nullptr)->m)))
 #endif
 #endif
 #elif (ACC_LIBC_FREESTANDING)
@@ -5771,9 +5771,9 @@ ACCLIB_EXTERN(int, acc_spawnve) (int mode, const char* fn, const char* const * a
     ACCCHK_ASSERT(sizeof(ptrdiff_t) >= sizeof(int))
     ACCCHK_ASSERT(sizeof(ptrdiff_t) >= sizeof(size_t))
 #if !(ACC_BROKEN_SIZEOF)
-    ACCCHK_ASSERT(sizeof(ptrdiff_t) == sizeof(ACC_STATIC_CAST(char*, 0) - ACC_STATIC_CAST(char*, 0)))
+    ACCCHK_ASSERT(sizeof(ptrdiff_t) == sizeof(ACC_STATIC_CAST(char*, nullptr) - ACC_STATIC_CAST(char*, nullptr)))
 # if (ACC_HAVE_MM_HUGE_PTR)
-    ACCCHK_ASSERT(4 == sizeof(ACC_STATIC_CAST(char __huge*, 0) - ACC_STATIC_CAST(char __huge*, 0)))
+    ACCCHK_ASSERT(4 == sizeof(ACC_STATIC_CAST(char __huge*, nullptr) - ACC_STATIC_CAST(char __huge*, nullptr)))
 # endif
 #endif
 #if (ACC_SIZEOF_PTRDIFF_T > 0)
diff --git a/src/pefile.cpp b/src/pefile.cpp
index 85da2936..1dfad1c1 100644
--- a/src/pefile.cpp
+++ b/src/pefile.cpp
@@ -151,8 +151,8 @@ int PeFile::readFileHeader()
 
     COMPILE_TIME_ASSERT(sizeof(exe_header_t) == 64)
     COMPILE_TIME_ASSERT_ALIGNED1(exe_header_t)
-    COMPILE_TIME_ASSERT(sizeof(((exe_header_t*)0)->_)  == 18)
-    COMPILE_TIME_ASSERT(sizeof(((exe_header_t*)0)->__) == 34)
+    COMPILE_TIME_ASSERT(sizeof(((exe_header_t*)nullptr)->_)  == 18)
+    COMPILE_TIME_ASSERT(sizeof(((exe_header_t*)nullptr)->__) == 34)
 
     exe_header_t h;
     int ic;
@@ -2096,8 +2096,8 @@ unsigned PeFile::stripDebug(unsigned overlaystart)
 
     COMPILE_TIME_ASSERT(sizeof(debug_dir_t) == 28)
     COMPILE_TIME_ASSERT_ALIGNED1(debug_dir_t)
-    COMPILE_TIME_ASSERT(sizeof(((debug_dir_t*)0)->_)  == 16)
-    COMPILE_TIME_ASSERT(sizeof(((debug_dir_t*)0)->__) ==  4)
+    COMPILE_TIME_ASSERT(sizeof(((debug_dir_t*)nullptr)->_)  == 16)
+    COMPILE_TIME_ASSERT(sizeof(((debug_dir_t*)nullptr)->__) ==  4)
 
     unsigned const take = IDSIZE(PEDIR_DEBUG);
     unsigned const skip = IDADDR(PEDIR_DEBUG);
@@ -2166,7 +2166,7 @@ void PeFile::checkHeaderValues(unsigned subsystem, unsigned mask,
     if (IDSIZE(PEDIR_COMRT))
         throwCantPack(".NET files are not yet supported");
 
-    if(isection == NULL)
+    if (isection == nullptr)
         throwCantPack("No section was found");
 
     if (memcmp(isection[0].name,"UPX",3) == 0)