diff --git a/packages/block-library/src/heading/index.js b/packages/block-library/src/heading/index.js
index f576098578568..5a12b6e7a8cc0 100644
--- a/packages/block-library/src/heading/index.js
+++ b/packages/block-library/src/heading/index.js
@@ -168,7 +168,7 @@ export const settings = {
 
 	merge( attributes, attributesToMerge ) {
 		return {
-			content: attributes.content + attributesToMerge.content,
+			content: ( attributes.content || '' ) + ( attributesToMerge.content || '' ),
 		};
 	},
 
diff --git a/packages/block-library/src/paragraph/index.js b/packages/block-library/src/paragraph/index.js
index 460bb680d3dcf..6a0c6172c3695 100644
--- a/packages/block-library/src/paragraph/index.js
+++ b/packages/block-library/src/paragraph/index.js
@@ -212,7 +212,7 @@ export const settings = {
 
 	merge( attributes, attributesToMerge ) {
 		return {
-			content: attributes.content + attributesToMerge.content,
+			content: ( attributes.content || '' ) + ( attributesToMerge.content || '' ),
 		};
 	},