sys:run-program errs on first invocation, works afterwards

On Windows 10, ABCL 1.4.0 sys:run-program first issues an error.

With abcl --no-init, the first call goes like this:
(sys:run-program "where.exe" '("mvn.bat"))

Error message:

Class not found: java.lang.UNIXProcess
   [Condition of type ERROR]

...
 48: (JCLASS "java.lang.UNIXProcess")
 49: (SYSTEM::GET-FASL-FUNCTION #<org.armedbear.lisp.FaslClassLoader
org.armedbear.lisp.FaslClassLoad.... {445C74}> 48)
 50: (SYSTEM:INIT-FASL :VERSION 42)
 51: (SYSTEM::%EVAL (SYSTEM:RUN-PROGRAM "where.exe" '("mvn.bat")))


Repeating the command the second time, and parsing the output (as done in
abcl-asdf:find-mvn) gives the expected result.

Mirko