diff --git a/heat/engine/constraint/common_constraints.py b/heat/engine/constraint/common_constraints.py
index 8fc056ab55..7883cf0d8a 100644
--- a/heat/engine/constraint/common_constraints.py
+++ b/heat/engine/constraint/common_constraints.py
@@ -13,6 +13,7 @@
 
 import croniter
 import eventlet
+import json
 import netaddr
 import zoneinfo
 
@@ -188,6 +189,19 @@ class ExpirationConstraint(constraints.BaseCustomConstraint):
             raise ValueError(_('Expiration time is out of date.'))
         except Exception as ex:
             self._error_message = (_(
-                'Expiration {0} is invalid: {1}').format(value,
-                                                         str(ex)))
+                'Expiration {0} is invalid: {1}').format(value, str(ex)))
+        return False
+
+
+class JsonStringConstraint(constraints.BaseCustomConstraint):
+
+    def validate(self, value, context):
+        if not value:
+            return True
+        try:
+            json.loads(value)
+            return True
+        except json.decoder.JSONDecodeError as ex:
+            self._error_message = (_(
+                'JSON string {0} is invalid: {1}').format(value, str(ex)))
         return False
diff --git a/heat/engine/resources/openstack/octavia/availability_zone_profile.py b/heat/engine/resources/openstack/octavia/availability_zone_profile.py
index a94880612c..28233a71db 100644
--- a/heat/engine/resources/openstack/octavia/availability_zone_profile.py
+++ b/heat/engine/resources/openstack/octavia/availability_zone_profile.py
@@ -12,6 +12,7 @@
 #    under the License.
 
 from heat.common.i18n import _
+from heat.engine import constraints
 from heat.engine import properties
 from heat.engine import resource
 from heat.engine import support
@@ -50,7 +51,8 @@ class AvailabilityZoneProfile(resource.Resource):
             properties.Schema.STRING,
             _('JSON string containing the availability zone metadata.'),
             update_allowed=True,
-            required=True
+            required=True,
+            constraints=[constraints.CustomConstraint('json_string')]
         ),
         PROVIDER_NAME: properties.Schema(
             properties.Schema.STRING,
diff --git a/heat/engine/resources/openstack/octavia/flavor_profile.py b/heat/engine/resources/openstack/octavia/flavor_profile.py
index ef20f6aca1..9a90cf32d2 100644
--- a/heat/engine/resources/openstack/octavia/flavor_profile.py
+++ b/heat/engine/resources/openstack/octavia/flavor_profile.py
@@ -12,6 +12,7 @@
 #    under the License.
 
 from heat.common.i18n import _
+from heat.engine import constraints
 from heat.engine import properties
 from heat.engine import resource
 from heat.engine import support
@@ -44,7 +45,8 @@ class FlavorProfile(resource.Resource):
             properties.Schema.STRING,
             _('JSON string containing the flavor metadata.'),
             update_allowed=True,
-            required=True
+            required=True,
+            constraints=[constraints.CustomConstraint('json_string')]
         ),
         PROVIDER_NAME: properties.Schema(
             properties.Schema.STRING,
diff --git a/heat/tests/constraints/test_common_constraints.py b/heat/tests/constraints/test_common_constraints.py
index 03399bf691..9af6e1dd06 100644
--- a/heat/tests/constraints/test_common_constraints.py
+++ b/heat/tests/constraints/test_common_constraints.py
@@ -445,3 +445,30 @@ class ExpirationConstraintTest(common.HeatTestCase):
 
     def test_validation_none(self):
         self.assertTrue(self.constraint.validate(None, self.ctx))
+
+
+class JsonStringConstraintTest(common.HeatTestCase):
+
+    def setUp(self):
+        super(JsonStringConstraintTest, self).setUp()
+        self.ctx = utils.dummy_context()
+        self.constraint = cc.JsonStringConstraint()
+
+    def test_validate_json(self):
+        data = '{"key": "value"}'
+        self.assertTrue(self.constraint.validate(data, None))
+
+    def test_validation_error(self):
+        data = '{\'key\': \'value\'}'
+        expected = (
+            "JSON string {0} is invalid: Expecting property name "
+            "enclosed in double quotes: line 1 column 2 (char 1)".format(data))
+
+        self.assertFalse(self.constraint.validate(data, self.ctx))
+        self.assertEqual(
+            expected,
+            str(self.constraint._error_message)
+        )
+
+    def test_validation_none(self):
+        self.assertTrue(self.constraint.validate(None, self.ctx))
diff --git a/setup.cfg b/setup.cfg
index 9b7626d235..2230a16163 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -107,6 +107,7 @@ heat.constraints =
     ip_or_cidr = heat.engine.constraint.common_constraints:IPCIDRConstraint
     test_constr = heat.engine.constraint.common_constraints:TestConstraintDelay
     timezone = heat.engine.constraint.common_constraints:TimezoneConstraint
+    json_string = heat.engine.constraint.common_constraints:JsonStringConstraint
     # service constraints
     barbican.container = heat.engine.clients.os.barbican:ContainerConstraint
     barbican.secret = heat.engine.clients.os.barbican:SecretConstraint