diff --git a/src/Psalm/Checker/Statements/Expression/Call/FunctionCallChecker.php b/src/Psalm/Checker/Statements/Expression/Call/FunctionCallChecker.php
index 4a7ea6f18..031bc6a15 100644
--- a/src/Psalm/Checker/Statements/Expression/Call/FunctionCallChecker.php
+++ b/src/Psalm/Checker/Statements/Expression/Call/FunctionCallChecker.php
@@ -216,6 +216,9 @@ class FunctionCallChecker extends \Psalm\Checker\Statements\Expression\CallCheck
 
             $is_predefined = true;
 
+            $is_maybe_root_function = !$stmt->name instanceof PhpParser\Node\Name\FullyQualified
+                && count($stmt->name->parts) === 1;
+
             if (!$in_call_map) {
                 $predefined_functions = $config->getPredefinedFunctions();
                 $is_predefined = isset($predefined_functions[$function_id]);
@@ -233,13 +236,18 @@ class FunctionCallChecker extends \Psalm\Checker\Statements\Expression\CallCheck
                     if (self::checkFunctionExists(
                         $statements_checker,
                         $function_id,
-                        $code_location
+                        $code_location,
+                        $is_maybe_root_function
                     ) === false
                     ) {
                         return false;
                     }
                 } else {
-                    $function_id = self::getExistingFunctionId($statements_checker, $function_id);
+                    $function_id = self::getExistingFunctionId(
+                        $statements_checker,
+                        $function_id,
+                        $is_maybe_root_function
+                    );
                 }
 
                 $function_exists = $is_stubbed || $codebase_functions->functionExists(
diff --git a/src/Psalm/Checker/Statements/Expression/CallChecker.php b/src/Psalm/Checker/Statements/Expression/CallChecker.php
index 110596de4..df7a0f937 100644
--- a/src/Psalm/Checker/Statements/Expression/CallChecker.php
+++ b/src/Psalm/Checker/Statements/Expression/CallChecker.php
@@ -1269,7 +1269,8 @@ class CallChecker
                             if (self::checkFunctionExists(
                                 $statements_checker,
                                 $function_id,
-                                $code_location
+                                $code_location,
+                                false
                             ) === false
                             ) {
                                 return false;
@@ -1377,13 +1378,15 @@ class CallChecker
      * @param  StatementsChecker    $statements_checker
      * @param  string               $function_id
      * @param  CodeLocation         $code_location
+     * @param  bool                 $can_be_in_root_scope if true, the function can be shortened to the root version
      *
      * @return bool
      */
     protected static function checkFunctionExists(
         StatementsChecker $statements_checker,
         &$function_id,
-        CodeLocation $code_location
+        CodeLocation $code_location,
+        $can_be_in_root_scope
     ) {
         $cased_function_id = $function_id;
         $function_id = strtolower($function_id);
@@ -1393,7 +1396,8 @@ class CallChecker
         if (!$codebase->functions->functionExists($statements_checker, $function_id)) {
             $root_function_id = preg_replace('/.*\\\/', '', $function_id);
 
-            if ($function_id !== $root_function_id
+            if ($can_be_in_root_scope
+                && $function_id !== $root_function_id
                 && $codebase->functions->functionExists($statements_checker, $root_function_id)
             ) {
                 $function_id = $root_function_id;
@@ -1418,11 +1422,15 @@ class CallChecker
     /**
      * @param  StatementsChecker    $statements_checker
      * @param  string               $function_id
+     * @param  bool                 $can_be_in_root_scope if true, the function can be shortened to the root version
      *
      * @return string
      */
-    protected static function getExistingFunctionId(StatementsChecker $statements_checker, $function_id)
-    {
+    protected static function getExistingFunctionId(
+        StatementsChecker $statements_checker,
+        $function_id,
+        $can_be_in_root_scope
+    ) {
         $function_id = strtolower($function_id);
 
         $codebase = $statements_checker->getFileChecker()->project_checker->codebase;
@@ -1431,6 +1439,10 @@ class CallChecker
             return $function_id;
         }
 
+        if (!$can_be_in_root_scope) {
+            return $function_id;
+        }
+
         $root_function_id = preg_replace('/.*\\\/', '', $function_id);
 
         if ($function_id !== $root_function_id
diff --git a/tests/NamespaceTest.php b/tests/NamespaceTest.php
index 7db1f9d67..daab95905 100644
--- a/tests/NamespaceTest.php
+++ b/tests/NamespaceTest.php
@@ -4,6 +4,7 @@ namespace Psalm\Tests;
 class NamespaceTest extends TestCase
 {
     use Traits\FileCheckerValidCodeParseTestTrait;
+    use Traits\FileCheckerInvalidCodeParseTestTrait;
 
     /**
      * @return array
@@ -16,17 +17,17 @@ class NamespaceTest extends TestCase
                     namespace A {
                         /** @return void */
                         function foo() {
-            
+
                         }
-            
+
                         class Bar {
-            
+
                         }
                     }
                     namespace {
                         A\foo();
                         \A\foo();
-            
+
                         (new A\Bar);
                     }',
             ],
@@ -40,7 +41,7 @@ class NamespaceTest extends TestCase
                         function foo() {
                             echo \Aye\Bee\HELLO;
                         }
-            
+
                         class Bar {
                             /** @return void */
                             public function foo() {
@@ -51,4 +52,39 @@ class NamespaceTest extends TestCase
             ],
         ];
     }
+
+    /**
+     * @return array
+     */
+    public function providerFileCheckerInvalidCodeParse()
+    {
+        return [
+            'callNamespacedFunctionFromEmptyNamespace' => [
+                '<?php
+                    namespace A {
+                        /** @return void */
+                        function foo() {
+
+                        }
+                    }
+                    namespace {
+                        foo();
+                    }',
+                'error_message' => 'UndefinedFunction',
+            ],
+            'callRootFunctionFromNamespace' => [
+                '<?php
+                    namespace {
+                        /** @return void */
+                        function foo() {
+
+                        }
+                    }
+                    namespace A {
+                        \A\foo();
+                    }',
+                'error_message' => 'UndefinedFunction',
+            ],
+        ];
+    }
 }