diff --git a/doc/CHANGES.md b/doc/CHANGES.md
index c38eb50..10f0b29 100644
--- a/doc/CHANGES.md
+++ b/doc/CHANGES.md
@@ -7,6 +7,15 @@ Change history
     we can no longer check for font feature existence before applying the OpenType tags.
     This release removes checking. (#515)
 
+  * Fix regression in loading font families constructed from fonts with different extensions;
+    e.g.: (#514)
+
+        \setmainfont{MyLato}
+          [
+            UprightFont       = Lato-Regular.ttf ,
+            BoldFont          = FiraSans-Bold.otf ,
+          ]
+
 
 ## v2.9c (2024/05/03)
 
diff --git a/fontspec-code-internal.dtx b/fontspec-code-internal.dtx
index 16e2922..74130d7 100644
--- a/fontspec-code-internal.dtx
+++ b/fontspec-code-internal.dtx
@@ -37,13 +37,11 @@
     \@@_font_suppress_not_found_error:
     \@@_init:
 
-    \@@_sanitise_fontname:Nn \l_fontspec_fontname_tl    {#2}
-    \@@_sanitise_fontname:Nn \l_@@_fontname_up_tl       {#2}
-    \@@_sanitise_fontname:Nn \l_@@_basename_tl          {#2}
+    \@@_sanitise_fontname:Nn \l_fontspec_fontname_tl {#2}
+    \tl_set_eq:NN \l_@@_fontname_up_tl \l_fontspec_fontname_tl
+    \tl_set_eq:NN \l_@@_basename_tl    \l_fontspec_fontname_tl
 
 %<debug>\typeout{^^J::::::::::::::: l_fontspec_fontname_tl~ =~ \l_fontspec_fontname_tl }
-%<debug>\typeout{::::::::::::::: _fontname_up_tl~ =~ \l_@@_fontname_up_tl }
-%<debug>\typeout{::::::::::::::: l_@@_extension_tl~ =~ \l_@@_extension_tl }
 
     \@@_if_detect_external:nT {#2}
      { \keys_set:nn {fontspec-preparse-external} {Path} }
@@ -107,6 +105,7 @@
       {
         \tl_if_in:NnT #1 {##1}
           {
+%<debug>  \typeout{::@@_process_ext:N~ --~ Removing~ EXT:~ ##1}
             \tl_remove_once:Nn #1 {##1}
             \tl_set:Nn \l_@@_extension_tl {##1}
             \@@_font_is_file:
@@ -143,9 +142,9 @@
   {
     \str_if_eq:eeT { \str_lowercase:f {\l_@@_extension_tl} } {.ttc}
       {
-        \@@_sanitise_fontname:Nn \l_@@_fontname_it_tl   {#1}
-        \@@_sanitise_fontname:Nn \l_@@_fontname_bf_tl   {#1}
-        \@@_sanitise_fontname:Nn \l_@@_fontname_bfit_tl {#1}
+        \tl_set_eq:NN \l_@@_fontname_it_tl   \l_fontspec_fontname_tl
+        \tl_set_eq:NN \l_@@_fontname_bf_tl   \l_fontspec_fontname_tl
+        \tl_set_eq:NN \l_@@_fontname_bfit_tl \l_fontspec_fontname_tl
       }
   }
 %    \end{macrocode}
@@ -253,7 +252,8 @@
   {
 %<debug>\typeout{:: @@_load_font}
 
-%<debug>\typeout{Set~ base~ font~ for~ preliminary~ analysis: \@@_construct_font_call:nn { \l_@@_fontname_up_tl } {} }
+    \@@_sanitise_fontname:Nn \l_@@_fontname_up_tl { \l_@@_fontname_up_tl }
+%<debug>\typeout{Set~ base~ font~ for~ preliminary~ analysis:~ "\l_@@_fontname_up_tl"~ with~ features~ "\l_@@_pre_feat_sclist" }
     \@@_primitive_font_set:NnnF \l_@@_test_font
       { \@@_construct_font_call:nn { \l_@@_fontname_up_tl } { \l_@@_pre_feat_sclist } }
       { \f@size pt - 2sp }
@@ -543,6 +543,7 @@
 %    \begin{macrocode}
 \cs_new:Nn \@@_set_faces_aux:nnnnn
   {
+%<debug>  \typeout{:: @@_set_faces_aux:nnnnn  \exp_not:n { {#1} {#2} {#3} {#4} {#5} } }
     \fontspec_complete_fontname:Nn \l_@@_curr_fontname_tl {#3}
     \@@_make_font_shapes:Nnnnn \l_@@_curr_fontname_tl {#1} {#2} {#4} {#5}
   }
@@ -558,7 +559,6 @@
   {
     \tl_set:Nx #1 {#2}
     \tl_replace_all:Nne #1 {*} {\l_@@_basename_tl}
-    \@@_process_ext:N #1
   }
 %    \end{macrocode}
 % \end{macro}
diff --git a/testfiles/fontload-ext-mixed.luatex.tlg b/testfiles/fontload-ext-mixed.luatex.tlg
new file mode 100644
index 0000000..4c491e5
--- /dev/null
+++ b/testfiles/fontload-ext-mixed.luatex.tlg
@@ -0,0 +1,15 @@
+This is a generated file for the l3build validation system.
+Don't change this file in any respect.
+LATO normally:
+TU/Lato-Regular.ttf(0)/m/n:
+  [Lato-Regular.ttf]:mode=node;script=latn;language=dflt;+tlig;
+FIRA normally:
+TU/FiraSans-Bold.otf(0)/m/n:
+  [FiraSans-Bold.otf]:mode=node;script=latn;language=dflt;+tlig;
+BOTH:
+TU/MyLato(0)/m/n:
+  [Lato-Regular.ttf]:mode=node;script=latn;language=dflt;+tlig;
+TU/MyLato(0)/b/n:
+  [FiraSans-Bold.otf]:mode=node;script=latn;language=dflt;+tlig;
+***************
+Compilation 1 of test file completed with exit status 0
diff --git a/testfiles/fontload-ext-mixed.lvt b/testfiles/fontload-ext-mixed.lvt
new file mode 100644
index 0000000..91037c5
--- /dev/null
+++ b/testfiles/fontload-ext-mixed.lvt
@@ -0,0 +1,26 @@
+
+\input{fontspec-testsetup.tex}
+
+\usepackage{fontspec}
+
+\begin{document}
+
+\MSG{LATO normally:}
+\setmainfont{Lato-Regular.ttf}
+hello \CURRNFSS
+
+\MSG{FIRA normally:}
+\setmainfont{FiraSans-Bold.otf}
+hello \CURRNFSS
+
+\MSG{BOTH:}
+\setmainfont{MyLato}
+  [
+    UprightFont = Lato-Regular.ttf  ,
+    BoldFont    = FiraSans-Bold.otf ,
+  ]
+hello \CURRNFSS
+\bfseries
+hello \CURRNFSS
+
+\end{document}
diff --git a/testfiles/fontload-ext-mixed.tlg b/testfiles/fontload-ext-mixed.tlg
new file mode 100644
index 0000000..e6d9521
--- /dev/null
+++ b/testfiles/fontload-ext-mixed.tlg
@@ -0,0 +1,15 @@
+This is a generated file for the l3build validation system.
+Don't change this file in any respect.
+LATO normally:
+TU/Lato-Regular.ttf(0)/m/n:
+  "[Lato-Regular.ttf]/OT:script=latn;language=dflt;mapping=tex-text;"
+FIRA normally:
+TU/FiraSans-Bold.otf(0)/m/n:
+  "[FiraSans-Bold.otf]/OT:script=latn;language=dflt;mapping=tex-text;"
+BOTH:
+TU/MyLato(0)/m/n:
+  "[Lato-Regular.ttf]/OT:script=latn;language=dflt;mapping=tex-text;"
+TU/MyLato(0)/b/n:
+  "[FiraSans-Bold.otf]/OT:script=latn;language=dflt;mapping=tex-text;"
+***************
+Compilation 1 of test file completed with exit status 0