diff --git a/index/gp/gprbuild/gprbuild-external.toml b/index/gp/gprbuild/gprbuild-external.toml
index 301a80eb2..cd347dfb4 100644
--- a/index/gp/gprbuild/gprbuild-external.toml
+++ b/index/gp/gprbuild/gprbuild-external.toml
@@ -9,6 +9,10 @@ kind = "version-output"
 version-regexp = "^GPRBUILD ([\\d\\.-]+).*|^GPRBUILD Community ([\\d\\.-]+).*"
 version-command = ["gprbuild", "--version"]
 
+# Neither macOS distribution (Homebrew, MacPorts) provides gprbuild.
 [[external]]
 kind = "system"
-origin = ["gprbuild"]
+[external.origin.'case(os)']
+"freebsd" = ["gprbuild"]
+"linux"   = ["gprbuild"]
+"windows" = ["gprbuild"]