diff --git a/configure b/configure
index b3f3abe..2beee31 100755
--- a/configure
+++ b/configure
@@ -3099,6 +3099,14 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# It's worth testing for this because it creates a very confusing error
+if test "$default_port" == ""; then
+	as_fn_error $? "Invalid empty string supplied for \$PGPORT or --with-pgport" "$LINENO" 5
+# This won't catch something like "PGPORT=11a" but that produces a pretty easy
+# to understand compile error.
+elif test "$default_port" -lt "1" -o "$default_port" -gt "65535"; then
+	as_fn_error $? "port must be between 1 and 65535" "$LINENO" 5
+fi
 
 #
 # '-rpath'-like feature can be disabled
diff --git a/configure.in b/configure.in
index 0bd90d7..54e9a16 100644
--- a/configure.in
+++ b/configure.in
@@ -164,6 +164,14 @@ but it's convenient if your clients have the right default compiled in.
 AC_DEFINE_UNQUOTED(DEF_PGPORT_STR, "${default_port}",
                    [Define to the default TCP port number as a string constant.])
 AC_SUBST(default_port)
+# It's worth testing for this because it creates a very confusing error
+if test "$default_port" == ""; then
+	AC_MSG_ERROR([Invalid empty string supplied for \$PGPORT or --with-pgport])
+# This won't catch something like "PGPORT=11a" but that produces a pretty easy
+# to understand compile error.
+elif test "$default_port" -lt "1" -o "$default_port" -gt "65535"; then
+	AC_MSG_ERROR([port must be between 1 and 65535])
+fi
 
 #
 # '-rpath'-like feature can be disabled