diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts
index 54bb2d6cc92..6dd69974ca2 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts
@@ -134,6 +134,11 @@ export default util.createRule<Options, MessageIds>({
      */
     function isPossiblyUsedBeforeAssigned(node: ts.Expression): boolean {
       const declaration = util.getDeclaration(checker, node);
+      if (!declaration) {
+        // don't know what the declaration is for some reason, so just assume the worst
+        return true;
+      }
+
       if (
         // non-strict mode doesn't care about used before assigned errors
         isStrictCompilerOptionEnabled(compilerOptions, 'strictNullChecks') &&
diff --git a/packages/eslint-plugin/src/util/types.ts b/packages/eslint-plugin/src/util/types.ts
index 98ae09b02c4..04eb720126a 100644
--- a/packages/eslint-plugin/src/util/types.ts
+++ b/packages/eslint-plugin/src/util/types.ts
@@ -140,8 +140,17 @@ export function isNullableType(
 export function getDeclaration(
   checker: ts.TypeChecker,
   node: ts.Expression,
-): ts.Declaration {
-  return checker.getSymbolAtLocation(node)!.declarations![0];
+): ts.Declaration | null {
+  const symbol = checker.getSymbolAtLocation(node);
+  if (!symbol) {
+    return null;
+  }
+  const declarations = symbol.declarations;
+  if (!declarations) {
+    return null;
+  }
+
+  return declarations[0];
 }
 
 /**
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts
index 7ed69a441f4..4c06e95a042 100644
--- a/packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts
+++ b/packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts
@@ -95,6 +95,21 @@ declare const str: string | null;
 
 foo(str!);
     `,
+    // https://github.com/typescript-eslint/typescript-eslint/issues/532
+    `
+declare function a(a: string): any;
+declare const b: string | null;
+class Mx {
+  @a(b!)
+  private prop = 1;
+}
+    `,
+    `
+class Mx {
+  @a(b!)
+  private prop = 1;
+}
+    `,
   ],
 
   invalid: [
@@ -280,5 +295,30 @@ class Foo {
         },
       ],
     },
+    // https://github.com/typescript-eslint/typescript-eslint/issues/532
+    {
+      code: `
+declare function a(a: string): any;
+const b = 'asdf';
+class Mx {
+  @a(b!)
+  private prop = 1;
+}
+      `,
+      output: `
+declare function a(a: string): any;
+const b = 'asdf';
+class Mx {
+  @a(b)
+  private prop = 1;
+}
+      `,
+      errors: [
+        {
+          messageId: 'unnecessaryAssertion',
+          line: 5,
+        },
+      ],
+    },
   ],
 });