psalm: Fatal error when using threads

When we run Psalm in one thread it works as expected:

$ vendor/bin/psalm --show-info=false --threads=1
Scanning files...
Analyzing files...

------------------------------
No errors found!
------------------------------

Checks took 20.19 seconds and used 852.012MB of memory
Psalm was able to infer types for 92.7393% of the codebase

However, when running in more than one, we see fatal errors:

$ vendor/bin/psalm --show-info=false --threads=2
Scanning files...
Child terminated without returning a serialized array - response type=boolean
Child terminated without returning a serialized array - response type=boolean
Child terminated with return code 0 and signal 11
Child terminated with return code 0 and signal 11
PHP Fatal error:  Uncaught TypeError: Argument 1 passed to Psalm\IssueBuffer::addIssues() must be of the type array, null given, called in /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Internal/Codebase/Scanner.php on line 437 and defined in /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/IssueBuffer.php:297
Stack trace:
#0 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Internal/Codebase/Scanner.php(437): Psalm\IssueBuffer::addIssues(NULL)
#1 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Internal/Codebase/Scanner.php(329): Psalm\Internal\Codebase\Scanner->scanFilePaths(2)
#2 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Codebase.php(385): Psalm\Internal\Codebase\Scanner->scanFiles(Object(Psalm\Internal\Codebase\ClassLikes), 2)
#3 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Internal/Analyzer/ProjectAnalyzer.php(414): Psalm\Codebase->scanFiles(2)
#4 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/ in /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/IssueBuffer.php on line 297

Fatal error: Uncaught TypeError: Argument 1 passed to Psalm\IssueBuffer::addIssues() must be of the type array, null given, called in /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Internal/Codebase/Scanner.php on line 437 and defined in /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/IssueBuffer.php:297
Stack trace:
#0 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Internal/Codebase/Scanner.php(437): Psalm\IssueBuffer::addIssues(NULL)
#1 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Internal/Codebase/Scanner.php(329): Psalm\Internal\Codebase\Scanner->scanFilePaths(2)
#2 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Codebase.php(385): Psalm\Internal\Codebase\Scanner->scanFiles(Object(Psalm\Internal\Codebase\ClassLikes), 2)
#3 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/Internal/Analyzer/ProjectAnalyzer.php(414): Psalm\Codebase->scanFiles(2)
#4 /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/ in /Users/jaikdean/Documents/zorro/vendor/vimeo/psalm/src/Psalm/IssueBuffer.php on line 297

This is a private, commercial project, so I’m unable to share the code to aid in debugging, but if there’s anything else I can run to get more useful information, please let me know.

Thanks for making Psalm, it’s a fantastic tool and has helped improved our code quality massively.