From 8a7763b924262a8c1388406792490ff3c79a38e6 Mon Sep 17 00:00:00 2001
From: Daniel Borcherding <human@borch.ai>
Date: Fri, 17 Apr 2026 17:43:30 -0600
Subject: [PATCH] fix: nil map assignment panics on zero-value Viper struct

---
 viper.go      | 12 ++++++++++++
 viper_test.go | 16 ++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/viper.go b/viper.go
index 2d5158cbd..f092f68da 100644
--- a/viper.go
+++ b/viper.go
@@ -1538,6 +1538,12 @@ func SetDefault(key string, value any) { v.SetDefault(key, value) }
 // SetDefault is case-insensitive for a key.
 // Default only used when no value is provided by the user via flag, config or ENV.
 func (v *Viper) SetDefault(key string, value any) {
+	if v.defaults == nil {
+		v.defaults = make(map[string]any)
+	}
+	if v.keyDelim == "" {
+		v.keyDelim = "."
+	}
 	// If alias passed in, then set the proper default
 	key = v.realKey(strings.ToLower(key))
 	value = toCaseInsensitiveValue(value)
@@ -1561,6 +1567,12 @@ func Set(key string, value any) { v.Set(key, value) }
 // Will be used instead of values obtained via
 // flags, config file, ENV, default, or key/value store.
 func (v *Viper) Set(key string, value any) {
+	if v.override == nil {
+		v.override = make(map[string]any)
+	}
+	if v.keyDelim == "" {
+		v.keyDelim = "."
+	}
 	// If alias passed in, then set the proper override
 	key = v.realKey(strings.ToLower(key))
 	value = toCaseInsensitiveValue(value)
diff --git a/viper_test.go b/viper_test.go
index 8b0232aee..40277837d 100644
--- a/viper_test.go
+++ b/viper_test.go
@@ -2718,3 +2718,19 @@ func skipWindows(t *testing.T) {
 		t.Skip("Skip test on Windows")
 	}
 }
+
+func TestZeroValueViperSetDefault(t *testing.T) {
+	v := &Viper{}
+	v.SetDefault("foo.bar", "baz")
+	if v.Get("foo.bar") != "baz" {
+		t.Fatalf("Expected baz, got %v", v.Get("foo.bar"))
+	}
+}
+
+func TestZeroValueViperSet(t *testing.T) {
+	v := &Viper{}
+	v.Set("foo.bar", "baz")
+	if v.Get("foo.bar") != "baz" {
+		t.Fatalf("Expected baz, got %v", v.Get("foo.bar"))
+	}
+}