diff --git a/script.h b/script.h
index b4c2aad..7df3151 100644
--- a/script.h
+++ b/script.h
@@ -19,7 +19,7 @@
 
 #include "list.h"
 
-#define GPIO_BANK_MAX	13 /* N */
+#define GPIO_BANK_MAX	14 /* N, (zero-based) index 13 */
 
 /** head of the data tree */
 struct script {
diff --git a/script_bin.c b/script_bin.c
index ffe9051..6a06684 100644
--- a/script_bin.c
+++ b/script_bin.c
@@ -269,9 +269,12 @@ static int decompile_section(void *bin, size_t bin_size,
 			} else if (gpio->port == 0xffff) {
 				; /* port:power */
 			} else if (gpio->port < 1 || gpio->port > GPIO_BANK_MAX) {
-				pr_err("%s: %s.%s: unknown GPIO port bank %c (%u)\n",
-				       filename, section->name, entry->name,
-				       'A'+gpio->port, gpio->port);
+				pr_err("%s: %s.%s: unknown GPIO port bank ",
+				       filename, section->name, entry->name);
+				char c = 'A' + gpio->port - 1;
+				if (c >= 'A' && c <= 'Z')
+					pr_err("%c ", c);
+				pr_err("(%u)\n", gpio->port);
 				goto failure;
 			}
 			v[0] = gpio->mul_sel;