diff --git a/tool/create_iso b/tool/create_iso
index 0b11b4ef0..c5c0458c0 100755
--- a/tool/create_iso
+++ b/tool/create_iso
@@ -38,7 +38,7 @@ gen_iso_image = mkisofs     -f -l -R -hide-rr-moved -jcharset utf-8 \
 
 $(ISOIMG) iso:
 	@$(call gen_iso_image, $(ISOIMG) $(ISODIR))
-	@which isohybrid > /dev/null && isohybrid $(ISOIMG)
+	@isohybrid $(ISOIMG)
 
 STRIP_FILES = $(wildcard genode/*) $(wildcard pistachio/*)
 
diff --git a/tool/run b/tool/run
index 4983205d7..82c2251c7 100755
--- a/tool/run
+++ b/tool/run
@@ -422,11 +422,10 @@ proc create_iso_image_from_run_dir { } {
 	exec rm -f "[run_dir].iso"
 
 	#
-	# The 'create_iso' tool returns a non-zero return code even if
-	# successful. So we ignore the return code here.
+	# The 'create_iso' write diagnostics to stderr, which are interpreted as
+	# execution failure by expect unless '-ignorestderr' is set on 'exec'.
 	#
-	catch { exec [genode_dir]/tool/create_iso iso ISO=[run_dir] }
-	if {![file exists "[run_dir].iso"]} {
+	if {[catch {exec -ignorestderr [genode_dir]/tool/create_iso iso ISO=[run_dir]} ]} {
 		puts stderr "Error: ISO image creation failed"
 		exit -5
 	}