diff --git a/libtiff/tif_config.vc.h b/libtiff/tif_config.vc.h
index 78f3b204..939594f8 100644
--- a/libtiff/tif_config.vc.h
+++ b/libtiff/tif_config.vc.h
@@ -116,7 +116,9 @@
 #  else
 #    define HAVE_SNPRINTF 1
 #  endif
-#  if _MSC_VER >= 1920 /* Visual Studio 2019 has strtoll/strtoull */
+#  define HAVE_STRTOL 1
+#  define HAVE_STRTOUL 1
+#  if _MSC_VER >= 1900 /* Visual Studio 2015 added strtoll/strtoull */
 #    define HAVE_STRTOLL 1
 #    define HAVE_STRTOULL 1
 #  endif
diff --git a/nmake.opt b/nmake.opt
index d59be640..79b2fbbb 100644
--- a/nmake.opt
+++ b/nmake.opt
@@ -109,12 +109,11 @@ CHECK_JPEG_YCBCR_SUBSAMPLING = 1
 ####################### Compiler related options. #######################
 #
 
-# If your MSVC does not provide strtol() and strtoul(), then these
-# should be set to 0.
+
+# Indicate if the compiler provides strtol/strtoul/strtoll/strtoull.
+# Users of MSVC++ 14.0 ("Visual Studio 2015") and later should set all of these to 1
 HAVE_STRTOL = 1
 HAVE_STRTOUL = 1
-
-# Users of MSVC 19.20 ("Visual Studio 2019") and later should set these to 1
 HAVE_STRTOLL = 0
 HAVE_STRTOULL = 0
 
diff --git a/port/libport.h b/port/libport.h
index 24e559b6..cb302ef4 100644
--- a/port/libport.h
+++ b/port/libport.h
@@ -36,16 +36,16 @@ int strcasecmp(const char *s1, const char *s2);
 #  define HAVE_GETOPT 1
 #endif
 
-#if defined(HAVE_STRTOL)
+#if !defined(HAVE_STRTOL)
 long strtol(const char *nptr, char **endptr, int base);
 #endif
-#if defined(HAVE_STRTOLL)
+#if !defined(HAVE_STRTOLL)
 long long strtoll(const char *nptr, char **endptr, int base);
 #endif
-#if defined(HAVE_STRTOUL)
+#if !defined(HAVE_STRTOUL)
 unsigned long strtoul(const char *nptr, char **endptr, int base);
 #endif
-#if defined(HAVE_STRTOULL)
+#if !defined(HAVE_STRTOULL)
 unsigned long long strtoull(const char *nptr, char **endptr, int base);
 #endif