diff --git a/rendercv/data_models.py b/rendercv/data_models.py
index 2d72506..4e61504 100644
--- a/rendercv/data_models.py
+++ b/rendercv/data_models.py
@@ -483,7 +483,8 @@ class PublicationEntry(RenderCVBaseModel):
         title="Authors",
         description="The authors of the publication in order as a list of strings.",
     )
-    doi: str = pydantic.Field(
+    doi: Optional[str] = pydantic.Field(
+        default=None,
         title="DOI",
         description="The DOI of the publication.",
         examples=["10.48550/arXiv.2310.03138"],
@@ -513,26 +514,32 @@ class PublicationEntry(RenderCVBaseModel):
 
     @pydantic.field_validator("doi")
     @classmethod
-    def check_doi(cls, doi: str) -> str:
+    def check_doi(cls, doi: Optional[str]) -> Optional[str]:
         """Check if the DOI exists in the DOI System."""
-        # see https://stackoverflow.com/a/60671292/18840665 for the explanation of the
-        # next line:
-        ssl._create_default_https_context = ssl._create_unverified_context  # type: ignore
+        if doi is not None:
+            # see https://stackoverflow.com/a/60671292/18840665 for the explanation of
+            # the next line:
+            ssl._create_default_https_context = ssl._create_unverified_context  # type: ignore
 
-        doi_url = f"http://doi.org/{doi}"
+            doi_url = f"http://doi.org/{doi}"
 
-        try:
-            urlopen(doi_url)
-        except HTTPError as err:
-            if err.code == 404:
-                raise ValueError("DOI cannot be found in the DOI System!")
+            try:
+                urlopen(doi_url)
+            except HTTPError as err:
+                if err.code == 404:
+                    raise ValueError("DOI cannot be found in the DOI System!")
 
         return doi
 
     @functools.cached_property
     def doi_url(self) -> str:
         """Return the URL of the DOI."""
-        return f"https://doi.org/{self.doi}"
+        # self.doi == "" is added because None values are replaced with "" in
+        # renderer.TemplatedFile class (to make templating easier)
+        if self.doi is None or self.doi == "":
+            return ""
+        else:
+            return f"https://doi.org/{self.doi}"
 
     @functools.cached_property
     def date_string(self) -> str:
diff --git a/schema.json b/schema.json
index 05ce0b9..1a47611 100644
--- a/schema.json
+++ b/schema.json
@@ -4,6 +4,10 @@
       "additionalProperties": false,
       "description": "This class is the data model of the theme options for the classic theme.",
       "properties": {
+        "theme": {
+          "const": "classic",
+          "title": "Theme"
+        },
         "font_size": {
           "default": "10pt",
           "description": "The font size of the CV. The default value is 10pt.",
@@ -109,10 +113,6 @@
           "description": "Page, section title, entry field, and highlights field margins.",
           "title": "Margins"
         },
-        "theme": {
-          "const": "classic",
-          "title": "Theme"
-        },
         "show_timespan_in": {
           "default": [],
           "description": "The time span will be shown in the date and location column in these sections. The input should be a list of section titles as strings (case-sensitive). The default value is an empty list, which means the time span will not be shown in any section.",
@@ -962,12 +962,17 @@
           "type": "array"
         },
         "doi": {
+          "default": null,
           "description": "The DOI of the publication.",
           "examples": [
             "10.48550/arXiv.2310.03138"
           ],
           "title": "DOI",
-          "type": "string"
+          "allOf": [
+            {
+              "type": "string"
+            }
+          ]
         },
         "date": {
           "default": "2020-01-01",
@@ -998,7 +1003,6 @@
       "required": [
         "title",
         "authors",
-        "doi",
         "date"
       ],
       "title": "PublicationEntry",
@@ -1008,6 +1012,10 @@
       "additionalProperties": false,
       "description": "This class is the data model of the theme options for the sb2nov theme.",
       "properties": {
+        "theme": {
+          "const": "sb2nov",
+          "title": "Theme"
+        },
         "font_size": {
           "default": "10pt",
           "description": "The font size of the CV. The default value is 10pt.",
@@ -1112,10 +1120,6 @@
           },
           "description": "Page, section title, entry field, and highlights field margins.",
           "title": "Margins"
-        },
-        "theme": {
-          "const": "sb2nov",
-          "title": "Theme"
         }
       },
       "required": [
@@ -1191,6 +1195,7 @@
     },
     "design": {
       "default": {
+        "theme": "classic",
         "font_size": "10pt",
         "page_size": "letterpaper",
         "color": "#004f90",
@@ -1226,7 +1231,6 @@
             "top": "0.3 cm"
           }
         },
-        "theme": "classic",
         "show_timespan_in": []
       },
       "description": "The design information of the CV. The default is the classic theme.",