diff --git a/src/intTest/kotlin/mastodon/account/AccountApiTest.kt b/src/intTest/kotlin/mastodon/account/AccountApiTest.kt
index a70400fb..77f785c9 100644
--- a/src/intTest/kotlin/mastodon/account/AccountApiTest.kt
+++ b/src/intTest/kotlin/mastodon/account/AccountApiTest.kt
@@ -141,10 +141,11 @@ class AccountApiTest {
         mockMvc
             .post("/api/v1/accounts") {
                 contentType = MediaType.APPLICATION_FORM_URLENCODED
-                param("username", "api-test-user-3")
+                param("password", "api-test-user-3")
                 with(csrf())
             }
-            .andExpect { status { isBadRequest() } }
+            .andDo { print() }
+            .andExpect { status { isUnprocessableEntity() } }
     }
 
     @Test
@@ -156,7 +157,7 @@ class AccountApiTest {
                 param("username", "api-test-user-4")
                 with(csrf())
             }
-            .andExpect { status { isBadRequest() } }
+            .andExpect { status { isUnprocessableEntity() } }
     }
 
     @Test
diff --git a/src/main/kotlin/dev/usbharu/hideout/mastodon/infrastructure/springweb/MastodonApiControllerAdvice.kt b/src/main/kotlin/dev/usbharu/hideout/mastodon/infrastructure/springweb/MastodonApiControllerAdvice.kt
index e27f1a27..d2ca2c6a 100644
--- a/src/main/kotlin/dev/usbharu/hideout/mastodon/infrastructure/springweb/MastodonApiControllerAdvice.kt
+++ b/src/main/kotlin/dev/usbharu/hideout/mastodon/infrastructure/springweb/MastodonApiControllerAdvice.kt
@@ -32,6 +32,7 @@ import org.slf4j.LoggerFactory
 import org.springframework.http.HttpStatus
 import org.springframework.http.ResponseEntity
 import org.springframework.validation.BindException
+import org.springframework.validation.FieldError
 import org.springframework.web.bind.annotation.ControllerAdvice
 import org.springframework.web.bind.annotation.ExceptionHandler
 
@@ -52,23 +53,43 @@ class MastodonApiControllerAdvice {
     @ExceptionHandler(BindException::class)
     fun handleException(ex: BindException): ResponseEntity<UnprocessableEntityResponse> {
         logger.debug("Failed bind entity.", ex)
-        val error = ex.bindingResult.fieldErrors
-        val message = error.map {
-            "${it.field} ${it.defaultMessage}"
-        }.joinToString(prefix = "Validation failed: ")
 
-        val details = error.associate {
-            it.field to UnprocessableEntityResponseDetails(
-                when (it.code) {
-                    "Email" -> "ERR_INVALID"
-                    "Pattern" -> "ERR_INVALID"
-                    else -> "ERR_INVALID"
-                },
-                it.defaultMessage
-            )
+        val details = mutableMapOf<String, MutableList<UnprocessableEntityResponseDetails>>()
+
+        ex.allErrors.forEach {
+            val defaultMessage = it.defaultMessage
+            when {
+                it is FieldError -> {
+                    val code = when (it.code) {
+                        "Email" -> "ERR_INVALID"
+                        "Pattern" -> "ERR_INVALID"
+                        else -> "ERR_INVALID"
+                    }
+                    details.getOrPut(it.field) {
+                        mutableListOf()
+                    }.add(UnprocessableEntityResponseDetails(code, defaultMessage.orEmpty()))
+                }
+
+                defaultMessage?.startsWith("Parameter specified as non-null is null:") == true -> {
+                    val parameter = defaultMessage.substringAfterLast("parameter ")
+
+                    details.getOrPut(parameter) {
+                        mutableListOf()
+                    }.add(UnprocessableEntityResponseDetails("ERR_BLANK", "can't be blank"))
+                }
+
+                else -> {
+                    logger.warn("Unknown validation error", ex)
+                }
+            }
         }
 
-        return ResponseEntity.unprocessableEntity().body(UnprocessableEntityResponse(message, details))
+        val message = details.map {
+            it.key + " " + it.value.joinToString { it.description }
+        }.joinToString()
+
+        return ResponseEntity.unprocessableEntity()
+            .body(UnprocessableEntityResponse(message, details))
     }
 
     @ExceptionHandler(StatusNotFoundException::class)
diff --git a/src/main/resources/openapi/mastodon.yaml b/src/main/resources/openapi/mastodon.yaml
index 66fb6704..e104eff7 100644
--- a/src/main/resources/openapi/mastodon.yaml
+++ b/src/main/resources/openapi/mastodon.yaml
@@ -1455,6 +1455,8 @@ components:
       type: object
       properties:
         username:
+
+          nullable: false
           type: string
           minLength: 1
           maxLength: 300
@@ -2841,9 +2843,11 @@ components:
         error:
           type: string
         details:
-          type: object
+          type: array
           additionalProperties:
-            $ref: "#/components/schemas/UnprocessableEntityResponseDetails"
+            type: array
+            items:
+              $ref: "#/components/schemas/UnprocessableEntityResponseDetails"
 
     UnprocessableEntityResponseDetails:
       type: object
@@ -2852,6 +2856,10 @@ components:
           type: string
         description:
           type: string
+          nullable: false
+      required:
+        - error
+        - description
 
     UnauthorizedResponse:
       type: object
diff --git a/templates/beanValidationModel.mustache b/templates/beanValidationModel.mustache
index 99635314..0d4b99fa 100644
--- a/templates/beanValidationModel.mustache
+++ b/templates/beanValidationModel.mustache
@@ -35,4 +35,5 @@ isLong set
 Not Integer, not Long => we have a decimal value!
 }}{{^isInteger}}{{^isLong}}{{#minimum}}
     @get:DecimalMin("{{.}}"){{/minimum}}{{#maximum}}
-    @get:DecimalMax("{{.}}"){{/maximum}}{{/isLong}}{{/isInteger}}
\ No newline at end of file
+    @get:DecimalMax("{{.}}"){{/maximum}}{{/isLong}}{{/isInteger}}
+
diff --git a/templates/dataClassReqVar.mustache b/templates/dataClassReqVar.mustache
index 1812c1cf..9ae7d3a9 100644
--- a/templates/dataClassReqVar.mustache
+++ b/templates/dataClassReqVar.mustache
@@ -1,4 +1,4 @@
 {{#useBeanValidation}}{{>beanValidation}}{{>beanValidationModel}}{{/useBeanValidation}}{{#swagger2AnnotationLibrary}}
     @Schema({{#example}}example = "{{#lambdaRemoveLineBreak}}{{#lambdaEscapeDoubleQuote}}{{{.}}}{{/lambdaEscapeDoubleQuote}}{{/lambdaRemoveLineBreak}}", {{/example}}required = true, {{#isReadOnly}}readOnly = {{{isReadOnly}}}, {{/isReadOnly}}description = "{{{description}}}"){{/swagger2AnnotationLibrary}}{{#swagger1AnnotationLibrary}}
-    @ApiModelProperty({{#example}}example = "{{#lambdaRemoveLineBreak}}{{#lambdaEscapeDoubleQuote}}{{{.}}}{{/lambdaEscapeDoubleQuote}}{{/lambdaRemoveLineBreak}}", {{/example}}required = true, {{#isReadOnly}}readOnly = {{{isReadOnly}}}, {{/isReadOnly}}value = "{{{description}}}"){{/swagger1AnnotationLibrary}}
+    @ApiModelProperty({{#example}}example = "{{#lambdaRemoveLineBreak}}{{#lambdaEscapeDoubleQuote}}{{{.}}}{{/lambdaEscapeDoubleQuote}}{{/lambdaRemoveLineBreak}}", {{/example}}required = true, {{#isReadOnly}}readOnly = {{{isReadOnly}}}, {{/isReadOnly}}value = "{{{description}}}"){{/swagger1AnnotationLibrary}}{{^isNullable}}@get:NotNull{{/isNullable}}
 @get:JsonProperty("{{{baseName}}}", required = true){{#isInherited}} override{{/isInherited}} {{>modelMutable}} {{{name}}}: {{#isEnum}}{{#isArray}}{{baseType}}<{{/isArray}}{{classname}}.{{{nameInCamelCase}}}{{#isArray}}>{{/isArray}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}{{#isNullable}}?{{/isNullable}}{{#defaultValue}} = {{{.}}}{{/defaultValue}}