diff --git a/config.yml b/config.yml
index 0046810e4e..b37b98cbdf 100644
--- a/config.yml
+++ b/config.yml
@@ -101,6 +101,8 @@ errors:
   - EXPECT_FOR_DELIMITER
   - EXPECT_IDENT_REQ_PARAMETER
   - EXPECT_IN_DELIMITER
+  - EXPECT_LPAREN_AFTER_NOT_LPAREN
+  - EXPECT_LPAREN_AFTER_NOT_OTHER
   - EXPECT_LPAREN_REQ_PARAMETER
   - EXPECT_MESSAGE
   - EXPECT_RBRACKET
diff --git a/src/prism.c b/src/prism.c
index c5f5744f58..eb7a4f7b81 100644
--- a/src/prism.c
+++ b/src/prism.c
@@ -19756,6 +19756,20 @@ parse_expression_prefix(pm_parser_t *parser, pm_binding_power_t binding_power, b
             pm_arguments_t arguments = { 0 };
             pm_node_t *receiver = NULL;
 
+            // If we do not accept a command call, then we also do not accept a
+            // not without parentheses. In this case we need to reject this
+            // syntax.
+            if (!accepts_command_call && !match1(parser, PM_TOKEN_PARENTHESIS_LEFT)) {
+                if (match1(parser, PM_TOKEN_PARENTHESIS_LEFT_PARENTHESES)) {
+                    pm_parser_err(parser, parser->previous.end, parser->previous.end + 1, PM_ERR_EXPECT_LPAREN_AFTER_NOT_LPAREN);
+                } else {
+                    accept1(parser, PM_TOKEN_NEWLINE);
+                    pm_parser_err_current(parser, PM_ERR_EXPECT_LPAREN_AFTER_NOT_OTHER);
+                }
+
+                return (pm_node_t *) pm_missing_node_create(parser, parser->current.start, parser->current.end);
+            }
+
             accept1(parser, PM_TOKEN_NEWLINE);
 
             if (accept1(parser, PM_TOKEN_PARENTHESIS_LEFT)) {
diff --git a/templates/src/diagnostic.c.erb b/templates/src/diagnostic.c.erb
index ce98dc5acd..9a30a57e3b 100644
--- a/templates/src/diagnostic.c.erb
+++ b/templates/src/diagnostic.c.erb
@@ -184,6 +184,8 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_ERR_EXPECT_FOR_DELIMITER]               = { "unexpected %s; expected a 'do', newline, or ';' after the 'for' loop collection", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_EXPECT_IDENT_REQ_PARAMETER]         = { "expected an identifier for the required parameter", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_EXPECT_IN_DELIMITER]                = { "expected a delimiter after the patterns of an `in` clause", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_EXPECT_LPAREN_AFTER_NOT_LPAREN]     = { "expected a `(` immediately after `not`", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_EXPECT_LPAREN_AFTER_NOT_OTHER]      = { "expected a `(` after `not`", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_EXPECT_LPAREN_REQ_PARAMETER]        = { "expected a `(` to start a required parameter", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_EXPECT_MESSAGE]                     = { "unexpected %s; expecting a message to send to the receiver", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_EXPECT_RBRACKET]                    = { "expected a matching `]`", PM_ERROR_LEVEL_SYNTAX },
diff --git a/test/prism/errors/command_calls_31.txt b/test/prism/errors/command_calls_31.txt
new file mode 100644
index 0000000000..e662b25444
--- /dev/null
+++ b/test/prism/errors/command_calls_31.txt
@@ -0,0 +1,17 @@
+true && not true
+            ^~~~ expected a `(` after `not`
+            ^~~~ unexpected 'true', expecting end-of-input
+
+true || not true
+            ^~~~ expected a `(` after `not`
+            ^~~~ unexpected 'true', expecting end-of-input
+
+true && not (true)
+           ^ expected a `(` immediately after `not`
+            ^ unexpected '(', expecting end-of-input
+
+true && not
+true
+^~~~ expected a `(` after `not`
+^~~~ unexpected 'true', expecting end-of-input
+