diff --git a/src/class-convertkit-resource-v4.php b/src/class-convertkit-resource-v4.php
index 95a9d2a..9a56a48 100644
--- a/src/class-convertkit-resource-v4.php
+++ b/src/class-convertkit-resource-v4.php
@@ -181,15 +181,15 @@ public function get_by_id( $id ) {
 	 */
 	public function get_by( $key, $value ) {
 
-		// Don't mutate the underlying resources, so multiple calls to get()
-		// with different order_by and order properties are supported.
-		$resources = $this->resources;
-
 		// Don't attempt sorting if no resources exist.
 		if ( ! $this->exist() ) {
-			return $resources;
+			return false;
 		}
 
+		// Don't mutate the underlying resources, so multiple calls to get()
+		// with different order_by and order properties are supported.
+		$resources = $this->resources;
+
 		foreach ( $resources as $id => $resource ) {
 			// Remove this resource if it doesn't have the array key.
 			if ( ! array_key_exists( $key, $resource ) ) {
@@ -332,6 +332,21 @@ public function exist() {
 			return false;
 		}
 
+		// If the resources are not an array, no resources exist.
+		if ( ! is_array( $this->resources ) ) {
+			return false;
+		}
+
+		// Ignore cached resources stored in the options table if they are a flat array
+		// that originates from between 1.6.0 and 1.9.5.2 of the Plugin
+		// i.e. [id => name], rather than [id => [id => '...', name => '...', ...]].
+		// The Plugin's upgrade routine must handle converting the flat array to the new array format, but the
+		// resource classes are typically loaded before this happens, so this check is necessary to avoid
+		// fatal errors when a user upgrades from 1.6.0...1.9.5.2 to 1.9.6 or higher.
+		if ( ! is_array( reset( $this->resources ) ) ) {
+			return false;
+		}
+
 		return ( count( $this->resources ) ? true : false );
 
 	}
diff --git a/tests/Integration/ResourceTest.php b/tests/Integration/ResourceTest.php
index 78e8f70..08b0f4e 100644
--- a/tests/Integration/ResourceTest.php
+++ b/tests/Integration/ResourceTest.php
@@ -258,6 +258,34 @@ public function testGetByMultipleValues()
 		$this->assertEquals('Z Name', end($result)[ $this->resource->order_by ]);
 	}
 
+	/**
+	 * Tests that the get_by() function returns false when queried with invalid resource data.
+	 *
+	 * @since   2.1.6
+	 */
+	public function testGetByWithInvalidResourceData()
+	{
+		// Mock invalid resource data.
+		$this->mockInvalidData();
+
+		// Assert result is false.
+		$this->assertFalse($this->resource->get_by('name', 'Z Name'));
+	}
+
+	/**
+	 * Tests that the exist() function returns false when queried with invalid resource data.
+	 *
+	 * @since   2.1.6
+	 */
+	public function testExistWithInvalidResourceData()
+	{
+		// Mock invalid resource data.
+		$this->mockInvalidData();
+
+		// Assert result is false.
+		$this->assertFalse($this->resource->exist());
+	}
+
 	/**
 	 * Tests that the refresh() function for Forms returns resources in an array, and that they are
 	 * in alphabetical ascending order by default.
@@ -669,4 +697,16 @@ private function mockData()
 			],
 		];
 	}
+
+	/**
+	 * Defines an array of resources when mocking invalid resource data in tests.
+	 *
+	 * @since   2.1.6
+	 */
+	private function mockInvalidData()
+	{
+		$this->resource->resources = [
+			2780977 => 'Resource',
+		];
+	}
 }