when passing a single, bare value to logger functions…

if the bare value is an error, use the key “error”

core.go

@@ -132,8 +132,10 @@ func (l *Core) sweetenFields(args []interface{}) []zap.Field {
 		if len(args) == 1 {
 			// passed a bare arg with no key.  We'll handle this
 			// as a special case
-			fields = append(fields, zap.Any("", args[0]))
-			return fields
+			if err, ok := args[0].(error); ok {
+				return append(fields, zap.Error(err))
+			}
+			return append(fields, zap.Any("", args[0]))
 		}
 
 		// Make sure this element isn't a dangling key.

core_test.go

@@ -0,0 +1,22 @@
+package flume
+
+import (
+	"errors"
+	"github.com/stretchr/testify/assert"
+	"go.uber.org/zap"
+	"testing"
+)
+
+func TestSweetenFields(t *testing.T) {
+
+	// single value, instead of key/value pairs
+	c := NewCore("asdf")
+	fields := c.sweetenFields([]interface{}{1})
+
+	assert.Equal(t, []zap.Field{zap.Int("", 1)}, fields)
+
+	// if the bare value is an error, use the key "error"
+	err := errors.New("blue")
+	fields = c.sweetenFields([]interface{}{err})
+	assert.Equal(t, []zap.Field{zap.NamedError("error", err)}, fields)
+}