diff --git a/common/bootcommand/boot_command_ast.go b/common/bootcommand/boot_command_ast.go
index 16b562e95..02ec5fbff 100644
--- a/common/bootcommand/boot_command_ast.go
+++ b/common/bootcommand/boot_command_ast.go
@@ -42,13 +42,22 @@ func (k KeyAction) String() string {
 }
 
 type expression interface {
+	// Do executes the expression
 	Do(context.Context, BCDriver) error
+	// Validate validates the expression without executing it
+	Validate() error
 }
 
 type expressionSequence []expression
 
 // Do executes every expression in the sequence and then finalizes the driver.
 func (s expressionSequence) Do(ctx context.Context, b BCDriver) error {
+	// validate should never fail here, since it should be called before
+	// expressionSequence.Do. Only reason we don't panic is so we can clean up.
+	if errs := s.Validate(); errs != nil {
+		return fmt.Errorf("Found an invalid boot command. This is likely an error in Packer, so please open a ticket.")
+	}
+
 	for _, exp := range s {
 		if err := ctx.Err(); err != nil {
 			return err
@@ -60,6 +69,16 @@ func (s expressionSequence) Do(ctx context.Context, b BCDriver) error {
 	return b.Finalize()
 }
 
+// Do executes every expression in the sequence and then finalizes the driver.
+func (s expressionSequence) Validate() (errs []error) {
+	for _, exp := range s {
+		if err := exp.Validate(); err != nil {
+			errs = append(errs, err)
+		}
+	}
+	return
+}
+
 // GenerateExpressionSequence generates a sequence of expressions from the
 // given command. This is the primary entry point to the boot command parser.
 func GenerateExpressionSequence(command string) (expressionSequence, error) {
@@ -67,9 +86,6 @@ func GenerateExpressionSequence(command string) (expressionSequence, error) {
 	if err != nil {
 		return nil, err
 	}
-	if got == nil {
-		return nil, fmt.Errorf("No expressions found.")
-	}
 	seq := expressionSequence{}
 	for _, exp := range got.([]interface{}) {
 		seq = append(seq, exp.(expression))
@@ -93,6 +109,14 @@ func (w *waitExpression) Do(ctx context.Context, _ BCDriver) error {
 	}
 }
 
+// Validate returns an error if the time is <= 0
+func (w *waitExpression) Validate() error {
+	if w.d <= 0 {
+		return fmt.Errorf("Expecting a positive wait value. Got %s", w.d)
+	}
+	return nil
+}
+
 func (w *waitExpression) String() string {
 	return fmt.Sprintf("Wait<%s>", w.d)
 }
@@ -107,6 +131,11 @@ func (s *specialExpression) Do(ctx context.Context, driver BCDriver) error {
 	return driver.SendSpecial(s.s, s.action)
 }
 
+// Validate always passes
+func (s *specialExpression) Validate() error {
+	return nil
+}
+
 func (s *specialExpression) String() string {
 	return fmt.Sprintf("Spec-%s(%s)", s.action, s.s)
 }
@@ -121,6 +150,11 @@ func (l *literal) Do(ctx context.Context, driver BCDriver) error {
 	return driver.SendKey(l.s, l.action)
 }
 
+// Validate always passes
+func (l *literal) Validate() error {
+	return nil
+}
+
 func (l *literal) String() string {
 	return fmt.Sprintf("LIT-%s(%s)", l.action, string(l.s))
 }
diff --git a/common/bootcommand/boot_command_ast_test.go b/common/bootcommand/boot_command_ast_test.go
index 8536f58c0..9638da42f 100644
--- a/common/bootcommand/boot_command_ast_test.go
+++ b/common/bootcommand/boot_command_ast_test.go
@@ -100,17 +100,41 @@ func Test_special(t *testing.T) {
 	}
 }
 
-func Test_negativeWait(t *testing.T) {
-	in := "<wait-1m>"
-	_, err := ParseReader("", strings.NewReader(in))
-	if err != nil {
-		log.Fatal(err)
+func Test_validation(t *testing.T) {
+	var expressions = []struct {
+		in    string
+		valid bool
+	}{
+		{
+			"<wait1m>",
+			true,
+		},
+		{
+			"<wait-1m>",
+			false,
+		},
+		{
+			"<f1>",
+			true,
+		},
+		{
+			"<",
+			true,
+		},
 	}
+	for _, tt := range expressions {
+		got, err := ParseReader("", strings.NewReader(tt.in))
+		if err != nil {
+			log.Fatal(err)
+		}
 
-	/*
 		gL := toIfaceSlice(got)
-			for _, g := range gL {
-				assert.Equal(t, tt.out, g.(*specialExpression).String())
-			}
-	*/
+		assert.Len(t, gL, 1)
+		err = gL[0].(expression).Validate()
+		if tt.valid {
+			assert.NoError(t, err)
+		} else {
+			assert.Error(t, err)
+		}
+	}
 }
diff --git a/common/bootcommand/config.go b/common/bootcommand/config.go
index 058efc2ed..ac5834701 100644
--- a/common/bootcommand/config.go
+++ b/common/bootcommand/config.go
@@ -33,6 +33,16 @@ func (c *BootConfig) Prepare(ctx *interpolate.Context) (errs []error) {
 			c.BootWait = bw
 		}
 	}
+
+	if c.BootCommand != nil {
+		expSeq, err := GenerateExpressionSequence(c.FlatBootCommand())
+		if err != nil {
+			errs = append(errs, err)
+		} else if vErrs := expSeq.Validate(); vErrs != nil {
+			errs = append(errs, vErrs...)
+		}
+	}
+
 	return
 }