diff --git a/format.cc b/format.cc
index bb486da6..a4ab143d 100644
--- a/format.cc
+++ b/format.cc
@@ -408,8 +408,12 @@ void Formatter::DoFormat() {
         ReportUnknownType(type, "string");
       const char *str = arg.string.value;
       size_t size = arg.string.size;
-      if (size == 0 && *str)
-        size = std::strlen(str);
+      if (size == 0) {
+        if (!str)
+          throw FormatError("string pointer is null");
+        if (*str)
+          size = std::strlen(str);
+      }
       char *out = GrowBuffer(std::max<size_t>(width, size));
       out = std::copy(str, str + size, out);
       if (static_cast<unsigned>(width) > size)
diff --git a/format_test.cc b/format_test.cc
index 1cdeb619..3f6d456b 100644
--- a/format_test.cc
+++ b/format_test.cc
@@ -609,6 +609,10 @@ TEST(FormatterTest, FormatCString) {
   CheckUnknownTypes("test", "s", "string");
   EXPECT_EQ("test", str(Format("{0}") << "test"));
   EXPECT_EQ("test", str(Format("{0:s}") << "test"));
+  char nonconst[] = "nonconst";
+  EXPECT_EQ("nonconst", str(Format("{0}") << nonconst));
+  EXPECT_THROW_MSG(Format("{0}") << reinterpret_cast<const char*>(0),
+      FormatError, "string pointer is null");
 }
 
 TEST(FormatterTest, FormatPointer) {