diff --git a/predicate.go b/predicate.go
index e377429..5b0dcc0 100644
--- a/predicate.go
+++ b/predicate.go
@@ -13,8 +13,11 @@ type Predicate func(last string) []Matcher
 // Or unions two predicate functions, so that the result predicate
 // returns the union of their predication
 func (p Predicate) Or(other Predicate) Predicate {
-	if p == nil || other == nil {
-		return nil
+	if p == nil {
+		return other
+	}
+	if other == nil {
+		return p
 	}
 	return func(last string) []Matcher { return append(p.predict(last), other.predict(last)...) }
 }
diff --git a/predicate_test.go b/predicate_test.go
new file mode 100644
index 0000000..d376d76
--- /dev/null
+++ b/predicate_test.go
@@ -0,0 +1,124 @@
+package complete
+
+import (
+	"os"
+	"sort"
+	"strings"
+	"testing"
+)
+
+func TestPredicate(t *testing.T) {
+	t.Parallel()
+
+	// Change to tests directory for testing completion of files and directories
+	err := os.Chdir("./tests")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tests := []struct {
+		name string
+		p    Predicate
+		arg  string
+		want []string
+	}{
+		{
+			name: "set",
+			p:    PredictSet("a", "b", "c"),
+			want: []string{"a", "b", "c"},
+		},
+		{
+			name: "set/empty",
+			p:    PredictSet(),
+			want: []string{},
+		},
+		{
+			name: "anything",
+			p:    PredictAnything,
+			want: []string{},
+		},
+		{
+			name: "nothing",
+			p:    PredictNothing,
+			want: []string{},
+		},
+		{
+			name: "or: word with nil",
+			p:    PredictSet("a").Or(PredictNothing),
+			want: []string{"a"},
+		},
+		{
+			name: "or: nil with word",
+			p:    PredictNothing.Or(PredictSet("a")),
+			want: []string{"a"},
+		},
+		{
+			name: "or: nil with nil",
+			p:    PredictNothing.Or(PredictNothing),
+			want: []string{},
+		},
+		{
+			name: "or: word with word with word",
+			p:    PredictSet("a").Or(PredictSet("b")).Or(PredictSet("c")),
+			want: []string{"a", "b", "c"},
+		},
+		{
+			name: "files/txt",
+			p:    PredictFiles("*.txt"),
+			want: []string{"./a.txt", "./b.txt", "./c.txt"},
+		},
+		{
+			name: "files/txt",
+			p:    PredictFiles("*.txt"),
+			arg:  "./dir/",
+			want: []string{},
+		},
+		{
+			name: "files/x",
+			p:    PredictFiles("x"),
+			arg:  "./dir/",
+			want: []string{"./dir/x"},
+		},
+		{
+			name: "files/*",
+			p:    PredictFiles("x*"),
+			arg:  "./dir/",
+			want: []string{"./dir/x"},
+		},
+		{
+			name: "files/md",
+			p:    PredictFiles("*.md"),
+			want: []string{"./readme.md"},
+		},
+		{
+			name: "dirs",
+			p:    PredictDirs,
+			arg:  "./dir/",
+			want: []string{"./dir"},
+		},
+		{
+			name: "dirs",
+			p:    PredictDirs,
+			want: []string{"./", "./dir"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name+"/"+tt.arg, func(t *testing.T) {
+			matchers := tt.p.predict(tt.arg)
+			matchersString := []string{}
+			for _, m := range matchers {
+				matchersString = append(matchersString, m.String())
+			}
+			sort.Strings(matchersString)
+			sort.Strings(tt.want)
+
+			got := strings.Join(matchersString, ",")
+			want := strings.Join(tt.want, ",")
+
+			if got != want {
+				t.Errorf("failed %s\ngot = %s\nwant: %s", tt.name, got, want)
+			}
+		})
+	}
+}
diff --git a/run_test.go b/run_test.go
index 4cbf36d..95b48b3 100644
--- a/run_test.go
+++ b/run_test.go
@@ -12,6 +12,8 @@ func TestCompleter_Complete(t *testing.T) {
 	// Set debug environment variable so logs will be printed
 	if testing.Verbose() {
 		os.Setenv(envDebug, "1")
+		// refresh the logger with environment variable set
+		Log = getLogger()
 	}
 
 	// Change to tests directory for testing completion of files and directories