diff --git a/src/fpm_environment.f90 b/src/fpm_environment.f90
index 7e8aa2317d..8e558e1c8e 100644
--- a/src/fpm_environment.f90
+++ b/src/fpm_environment.f90
@@ -50,6 +50,8 @@ integer function get_os_type() result(r)
 
         first_run = .false.
         r = OS_UNKNOWN
+        length = 0
+        rc = 0
 
         ! Check environment variable `OSTYPE`.
         call get_environment_variable('OSTYPE', val, length, rc)