diff --git a/src/userinput.c b/src/userinput.c
index ec19dbe2..250ece99 100644
--- a/src/userinput.c
+++ b/src/userinput.c
@@ -171,9 +171,22 @@ static int pinentry_exchange(int to, int from, char **retstr,
                              const char *format, ...)
 {
 	va_list ap;
+	char buffer[2048];
+	int size;
 
 	va_start(ap, format);
-	if (vdprintf(to, format, ap) == 0) {
+
+	size = vsnprintf(buffer, sizeof(buffer), format, ap);
+	if (size < 0) {
+		if (retstr)
+			*retstr = strdup(strerror(errno));
+		va_end(ap);
+		return -1;
+	}
+	if (size > sizeof(buffer))
+		size = sizeof(buffer);
+
+	if (write(to, buffer, size) < 0) {
 		if (retstr)
 			*retstr = strdup(strerror(errno));
 		va_end(ap);