diff --git a/commentaries/forms.py b/commentaries/forms.py
index fd158ef1e844320d32828170c0e609905bcb846e..be071a9fb1b38374e6baf30d1d51a93c7166d0dc 100644
--- a/commentaries/forms.py
+++ b/commentaries/forms.py
@@ -15,6 +15,7 @@ import strings
 class DOIToQueryForm(forms.Form):
     VALID_DOI_REGEXP = r'^(?i)10.\d{4,9}/[-._;()/:A-Z0-9]+$'
     doi = forms.RegexField(regex=VALID_DOI_REGEXP, strip=True, help_text=strings.doi_query_help_text,
+        error_messages={'invalid': strings.doi_query_invalid},
         widget=forms.TextInput({'label': 'DOI', 'placeholder': strings.doi_query_placeholder}))
 
     def clean_doi(self):
@@ -44,8 +45,8 @@ class ArxivQueryForm(forms.Form):
     VALID_ARXIV_IDENTIFIER_REGEX = "(?:{})|(?:{})".format(IDENTIFIER_PATTERN_NEW, IDENTIFIER_PATTERN_OLD)
 
     identifier = forms.RegexField(regex=VALID_ARXIV_IDENTIFIER_REGEX, strip=True,
-        help_text=strings.arxiv_query_help_text, widget=forms.TextInput(
-        {'placeholder': strings.arxiv_query_placeholder}))
+        help_text=strings.arxiv_query_help_text, error_messages={'invalid': strings.arxiv_query_invalid},
+        widget=forms.TextInput( {'placeholder': strings.arxiv_query_placeholder}))
 
     def clean_identifier(self):
         identifier = self.cleaned_data['identifier']
diff --git a/strings/__init__.py b/strings/__init__.py
index 67211441f87a16a389af55fef305130bd0ad69bb..51115395b4c01978c62b95c036d3f9a5e7445cfc 100644
--- a/strings/__init__.py
+++ b/strings/__init__.py
@@ -18,6 +18,10 @@ doi_query_help_text = (
     'For published papers, you can prefill the form (except for domain, subject area and abstract) using the DOI. '
     "(Give the DOI as 10.[4 to 9 digits]/[string], without prefix, as per the placeholder)."
 )
+doi_query_invalid = (
+    "DOI does not match the expression supplied by CrossRef. Either it is very old or you made a mistake. "
+    "If you are sure it is correct, please enter the metadata manually. Sorry for the inconvenience."
+)
 
 arxiv_query_placeholder = (
     "new style: YYMM.####(#)v#(#) or "
@@ -27,6 +31,7 @@ arxiv_query_help_text =  (
     "For preprints, you can prefill the form using the arXiv identifier. "
     "Give the identifier without prefix and do not forget the version number, as per the placeholder."
 )
+arxiv_query_invalid = 'ArXiv identifier is invalid. Did you include a version number?'
 
 # Arxiv response is not valid
 arxiv_caller_errormessages = {