diff --git a/codegen/src/main/kotlin/apifi/parser/PathsParser.kt b/codegen/src/main/kotlin/apifi/parser/PathsParser.kt
index 95aaf3d..786f97c 100644
--- a/codegen/src/main/kotlin/apifi/parser/PathsParser.kt
+++ b/codegen/src/main/kotlin/apifi/parser/PathsParser.kt
@@ -1,6 +1,7 @@
 package apifi.parser
 
 import apifi.helpers.replaceArrayToList
+import apifi.helpers.toCamelCase
 import apifi.helpers.toTitleCase
 import apifi.helpers.typeDeclaration
 import apifi.models.Model
@@ -21,7 +22,7 @@ object PathsParser {
                 val responses = ResponseBodyParser.parse(operation.responses, operationSpecifier)
                 models.addAll(request?.models ?: emptyList())
                 models.addAll(responses?.models ?: emptyList())
-                Operation(httpMethod, operation.operationId ?: httpMethod.toString().toLowerCase(),
+                Operation(httpMethod, getValidOperationId(operation.operationId) ?: httpMethod.toString().toLowerCase(),
                     operation.tags ?: emptyList(), params, request?.result, responses?.result ?: emptyList())
             }
             Path(endpoint, operations)
@@ -31,4 +32,17 @@ object PathsParser {
     private fun operationSpecifier(operation: io.swagger.v3.oas.models.Operation, httpMethod: PathItem.HttpMethod, endpoint: String) =
         (operation.operationId
             ?: (httpMethod.toString() + endpoint.replace(Regex("[^A-Za-z ]"), " ")).toTitleCase())
-}
\ No newline at end of file
+
+    /**
+     * Must start with a lower case letter and use the camel case and no underscores.
+     */
+    private fun getValidOperationId(operationId: String?): String? {
+        return operationId?.let {
+            if (it.first().isDigit()) {
+                "operation$operationId".toCamelCase()
+            } else {
+                operationId.toCamelCase()
+            }
+        }
+    }
+}
diff --git a/codegen/src/test-res/parser/params/with-invalid-operationId.yml b/codegen/src/test-res/parser/params/with-invalid-operationId.yml
new file mode 100644
index 0000000..ff447c0
--- /dev/null
+++ b/codegen/src/test-res/parser/params/with-invalid-operationId.yml
@@ -0,0 +1,16 @@
+openapi: "3.0.0"
+paths:
+  /pets:
+    get:
+      summary: List all pets
+      operationId: '1 list All_Pets '
+      parameters:
+        - name: id
+          in: query
+          description: id to filter by
+          required: false
+          style: form
+          schema:
+            type: array
+            items:
+              type: string
diff --git a/codegen/src/test/kotlin/apifi/parser/PathsParserTest.kt b/codegen/src/test/kotlin/apifi/parser/PathsParserTest.kt
index f4a97e4..5c6838a 100644
--- a/codegen/src/test/kotlin/apifi/parser/PathsParserTest.kt
+++ b/codegen/src/test/kotlin/apifi/parser/PathsParserTest.kt
@@ -5,6 +5,7 @@ import apifi.parser.models.Param
 import apifi.parser.models.ParamType
 import io.kotest.core.spec.style.DescribeSpec
 import io.kotest.matchers.shouldBe
+import io.kotest.matchers.shouldNotBe
 import io.swagger.v3.oas.models.PathItem.HttpMethod
 import io.swagger.v3.parser.OpenAPIV3Parser
 import org.apache.commons.io.FileUtils
@@ -57,5 +58,13 @@ class PathsParserTest : DescribeSpec({
             path.operations!![0].type shouldBe HttpMethod.GET
             path.operations!![0].params[0] shouldBe Param("id", "kotlin.collections.List<kotlin.String>", false, ParamType.Query)
         }
+        it("with invalid operationId") {
+            val file = FileUtils.getFile("src", "test-res", "parser", "params", "with-invalid-operationId.yml").readText().trimIndent()
+            val openApi = OpenAPIV3Parser().readContents(file).openAPI
+            val path = PathsParser.parse(openApi.paths).result[0]
+            val operation = path.operations?.first()
+            operation shouldNotBe null
+            operation?.name shouldBe "operation1ListAllPets"
+        }
     }
-})
\ No newline at end of file
+})