diff --git a/instrumenter.go b/instrumenter.go
index 999ebce..d635713 100644
--- a/instrumenter.go
+++ b/instrumenter.go
@@ -467,9 +467,7 @@ func (i *instrumenter) replace(n ast.Node) bool {
 // related to the instrumented condition. Especially for switch statements, the
 // position may differ from the expression that is wrapped.
 func (i *instrumenter) callCover(expr ast.Expr, pos token.Pos, code string) ast.Expr {
-	if !pos.IsValid() {
-		panic("pos must refer to the code from before instrumentation")
-	}
+	assert(pos.IsValid(), "pos must refer to the code from before instrumentation")
 
 	start := i.fset.Position(pos)
 	if !strings.HasSuffix(start.Filename, ".go") {
@@ -551,9 +549,7 @@ func (i *instrumenter) instrumentTestMain(astFile *ast.File) {
 				i.hasTestMain = true
 
 				ast.Inspect(decl.Body, wrapOsExit)
-				if !seenOsExit {
-					panic("gobco: can only handle TestMain with explicit call to os.Exit")
-				}
+				assert(seenOsExit, "can only handle TestMain with explicit call to os.Exit")
 			}
 		}
 	}
@@ -886,9 +882,3 @@ func shouldBuild(filename string) bool {
 	ok(err)
 	return m
 }
-
-func ok(err error) {
-	if err != nil {
-		panic(err)
-	}
-}
diff --git a/main.go b/main.go
index c7bc4cf..85eb89d 100644
--- a/main.go
+++ b/main.go
@@ -117,9 +117,7 @@ func (g *gobco) parseArgs(args []string) {
 		args = []string{"."}
 	}
 
-	if len(args) > 1 {
-		panic("gobco: checking multiple packages doesn't work yet")
-	}
+	assert(len(args) <= 1, "checking multiple packages doesn't work yet")
 
 	for _, arg := range args {
 		arg = filepath.FromSlash(arg)
diff --git a/util.go b/util.go
index f6386e8..b5daddf 100644
--- a/util.go
+++ b/util.go
@@ -94,3 +94,15 @@ func (s *sliceFlag) Set(str string) error {
 	*s.values = append(*s.values, str)
 	return nil
 }
+
+func ok(err error) {
+	if err != nil {
+		panic(err)
+	}
+}
+
+func assert(cond bool, msg string) {
+	if !cond {
+		panic(msg)
+	}
+}