diff --git a/scripts/config.py b/scripts/config.py
index 58be3b289..e01b9d541 100755
--- a/scripts/config.py
+++ b/scripts/config.py
@@ -302,10 +302,22 @@ class ConfigFile(Config):
         where <middle> is "#define <name> ".
         """
         setting = self.settings[name]
+        value = setting.value
+        if value is None:
+            value = ''
+        # Normally the whitespace to separte the symbol name from the
+        # value is part of middle, and there's no whitespace for a symbol
+        # with no value. But if a symbol has been changed from having a
+        # value to not having one, the whitespace is wrong, so fix it.
+        if value:
+            if middle[-1] not in '\t ':
+                middle += ' '
+        else:
+            middle = middle.rstrip()
         return ''.join([indent,
                         '' if setting.active else '//',
                         middle,
-                        setting.value]).rstrip()
+                        value]).rstrip()
 
     def write_to_stream(self, output):
         """Write the whole configuration to output."""
diff --git a/tests/scripts/test_config_script.py b/tests/scripts/test_config_script.py
index a71b35792..dd3ecbbdb 100755
--- a/tests/scripts/test_config_script.py
+++ b/tests/scripts/test_config_script.py
@@ -129,6 +129,7 @@ def run_one(options, args):
 TEST_SYMBOLS = [
     'CUSTOM_OPTION',
     'MBEDTLS_AES_C',
+    'MBEDTLS_MPI_MAX_SIZE',
     'MBEDTLS_NO_UDBL_DIVISION',
     'MBEDTLS_PLATFORM_ZEROIZE_ALT',
 ]
@@ -136,6 +137,7 @@ TEST_SYMBOLS = [
 ### A list of symbols to test with set with a value.
 TEST_SYMBOLS_WITH_VALUE = [
     'CUSTOM_VALUE',
+    'MBEDTLS_AES_C',
     'MBEDTLS_MPI_MAX_SIZE',
 ]
 
@@ -151,6 +153,7 @@ def run_all(options):
     for symbol in TEST_SYMBOLS_WITH_VALUE:
         run_one(options, ['set', symbol, 'value'])
         run_one(options, ['--force', 'set', symbol, 'value'])
+        run_one(options, ['unset', symbol])
 
 def main():
     """Command line entry point."""