diff --git a/protonkey b/protonkey
index 8ac2ecb..efd832d 100755
--- a/protonkey
+++ b/protonkey
@@ -17,21 +17,25 @@
 # along with this program. If not, see https://www.gnu.org/licenses/.
 
 if ! test -n "$1"; then
-	printf "Usage: %s [ProtonMail address]\n" "$0"
+	printf "Usage: %s [ProtonMail address]\n" "$0" 1>&2
 	exit 64 # syexits(3) EX_USAGE
 fi
 
-if ! command -v curl > /dev/null; then
-	printf "%s: Missing dependency: curl(1)\n" "$0"
-	exit 69 # sysexits(3) EX_UNAVAILABLE
-fi
+for command in \
+	curl \
+	gpg
+do
+	if ! command -v "$command" > /dev/null; then
+		printf "%s: Missing dependency: %s(1)\n" "$0" "$command" 1>&2
+		exit 69 # sysexits(3) EX_UNAVAILABLE
+	fi
+done
 
-ADDR="$1"
-URL="https://api.protonmail.ch/pks/lookup?op=get&search=$ADDR"
+URL="https://api.protonmail.ch/pks/lookup?op=get&search=$1"
 RESPONSE="$(curl -s "$URL")"
 
 if ! printf "%s\n" "$RESPONSE" | gpg --import >/dev/null 2>&1; then
-	printf "%s\n" "$RESPONSE"
+	printf "%s\n" "$RESPONSE" 1>&2
 	exit 76 # sysexits(3) EX_PROTOCOL
 else
 	exit 0