bkpr-report: make ? work as expected on amount fields.

diff --git a/contrib/msggen/msggen/schema.json b/contrib/msggen/msggen/schema.json
index e1f02594..0dbe3289 100644
--- a/contrib/msggen/msggen/schema.json
+++ b/contrib/msggen/msggen/schema.json
@@ -5162,7 +5162,7 @@
               "",
               "If a field is unavailable, it expands to an empty string.",
               "",
-              "You can provide fallback with ?, including more variable:",
+              "You can provide fallback with ?, which is used when the tag is not present (or zero, for credit, debit, fees and creditdebit).  This fallback can include more tags:",
               " * {outpoint?NONE}",
               " * {payment_id?txid: {txid?UNKNOWN}}",
               "The first one the outpoint, or NONE if that is not available. ",
diff --git a/doc/schemas/bkpr-report.json b/doc/schemas/bkpr-report.json
index 2eed4048..27d69c1a 100644
--- a/doc/schemas/bkpr-report.json
+++ b/doc/schemas/bkpr-report.json
@@ -48,7 +48,7 @@
           "",
           "If a field is unavailable, it expands to an empty string.",
           "",
-          "You can provide fallback with ?, including more variable:",
+          "You can provide fallback with ?, which is used when the tag is not present (or zero, for credit, debit, fees and creditdebit).  This fallback can include more tags:",
           " * {outpoint?NONE}",
           " * {payment_id?txid: {txid?UNKNOWN}}",
           "The first one the outpoint, or NONE if that is not available. ",
diff --git a/plugins/bkpr/report.c b/plugins/bkpr/report.c
index 754f29a8..fa4d4fa6 100644
--- a/plugins/bkpr/report.c
+++ b/plugins/bkpr/report.c
@@ -15,6 +15,9 @@
 #include <plugins/bkpr/report.h>
 #include <plugins/libplugin.h>
 
+/* This is a zero, but treated specially if tested with ? */
+static const char ZERO_AMOUNT[] = "0";
+
 static const char *report_fmt_acct_name(const tal_t *ctx UNNEEDED,
 					const struct bkpr *bkpr UNNEEDED,
 					const struct income_event *e)
@@ -40,6 +43,8 @@ static const char *report_fmt_credit(const tal_t *ctx,
 				     const struct bkpr *bkpr UNNEEDED,
 				     const struct income_event *e)
 {
+	if (amount_msat_is_zero(e->credit))
+		return ZERO_AMOUNT;
 	return fmt_amount_msat_btc(ctx, e->credit, false);
 }
 
@@ -47,6 +52,8 @@ static const char *report_fmt_debit(const tal_t *ctx,
 				    const struct bkpr *bkpr UNNEEDED,
 				    const struct income_event *e)
 {
+	if (amount_msat_is_zero(e->debit))
+		return ZERO_AMOUNT;
 	return fmt_amount_msat_btc(ctx, e->debit, false);
 }
 
@@ -54,6 +61,8 @@ static const char *report_fmt_fees(const tal_t *ctx,
 				   const struct bkpr *bkpr UNNEEDED,
 				   const struct income_event *e)
 {
+	if (amount_msat_is_zero(e->fees))
+		return ZERO_AMOUNT;
 	return fmt_amount_msat_btc(ctx, e->fees, false);
 }
 
@@ -134,7 +143,7 @@ static const char *report_fmt_credit_debit(const tal_t *ctx,
 	if (!amount_msat_is_zero(e->debit))
 		return tal_fmt(ctx, "-%s",
 			       fmt_amount_msat_btc(tmpctx, e->debit, false));
-	return "0";
+	return ZERO_AMOUNT;
 }
 
 static const char *report_fmt_currency_credit(const tal_t *ctx,
@@ -411,6 +420,10 @@ static char *format_event(const tal_t *ctx,
 		}
 
 		v = fmt->fmt[i](tmpctx, bkpr, e);
+		/* If there's an alternative, we treat ZERO_AMOUNT as missing. */
+		if (v == ZERO_AMOUNT && fmt->alt[i])
+			v = NULL;
+
 		if (v) {
 			v = escape_value(tmpctx, v, esc);
 			out = tal_strcat(ctx, take(out), v);
diff --git a/tests/test_bookkeeper.py b/tests/test_bookkeeper.py
index c62210e6..fb6b23b3 100644
--- a/tests/test_bookkeeper.py
+++ b/tests/test_bookkeeper.py
@@ -1275,22 +1275,24 @@ def test_bkpr_report_tags_and_fallback(node_factory):
     assert any(r[2] == "NONE" or r[3] == "NONE" or r[4] == "NONE" for r in rows)
 
     # Fancier fields should work, too
-    res = l1.rpc.bkpr_report(format="{tag}|{account}|{credit}|{debit}|{creditdebit}|{currencycredit}|{currencydebit}|{currencycreditdebit}")
+    res = l1.rpc.bkpr_report(format="{tag}|{account}|{credit}|{debit}|{creditdebit}|{currencycredit}|{currencydebit}|{currencycreditdebit}|{credit?NONE}")
     rows = [line.split("|") for line in res["report"]]
 
     for r in rows:
-        assert len(r) == 8
+        assert len(r) == 9
         # Credit or debit?
         if float(r[2]) > 0:
             assert r[4] == '+' + r[2]
             assert float(r[5]) > 0
             assert r[6] == '0.00'
             assert r[7] == '+' + r[5]
+            assert r[8] == r[2]
         else:
             assert r[4] == '-' + r[3]
             assert float(r[6]) > 0
             assert r[5] == '0.00'
             assert r[7] == '-' + r[6]
+            assert r[8] == 'NONE'
 
 
 def test_bkpr_report_invoice(node_factory):