diff --git a/examples/grid/material.yaml b/examples/grid/material.yaml
index ef35f9281..a89068e1b 100644
--- a/examples/grid/material.yaml
+++ b/examples/grid/material.yaml
@@ -6,29 +6,29 @@ homogenization:
 
 phase:
   Aluminum:
-    "lattice": 'cF'
+    lattice: cF
     mechanical:
       output: [F, P, F_e, F_p, L_p, O]
-      elastic: {type: "Hooke", C_11: 106.75e+9, C_12: 60.41e+9, C_44: 28.34e+9}
+      elastic: {type: Hooke, C_11: 106.75e+9, C_12: 60.41e+9, C_44: 28.34e+9}
       plastic:
-        'type': "phenopowerlaw"
-        "N_sl": [12]
+        type: phenopowerlaw
+        N_sl: [12]
         a_sl: 2.25
         atol_xi: 1.0
         dot_gamma_0_sl: 0.001
         h_0_sl-sl: 75.e+6
         h_sl-sl: [1, 1, 1.4, 1.4, 1.4, 1.4, 1.4]
         n_sl: 20
-        'output': ['xi_sl']
+        output: [xi_sl]
         xi_0_sl: [31.e+6]
         xi_inf_sl: [63.e+6]
 
 material:
   - homogenization: SX
     constituents:
-      - 'phase': "Aluminum"
-        'v': 1.0
-        "O": [1.0, 0.0, 0.0, 0.0]
+      - phase: Aluminum
+        v: 1.0
+        O: [1.0, 0.0, 0.0, 0.0]
   - homogenization: SX
     constituents:
       - phase: Aluminum
diff --git a/src/IO.f90 b/src/IO.f90
index 399e2e6df..16d385106 100644
--- a/src/IO.f90
+++ b/src/IO.f90
@@ -494,6 +494,8 @@ subroutine IO_error(error_ID,el,ip,g,instance,ext_msg)
       msg = '--- expected after YAML file header'
     case (709)
       msg = 'Length mismatch'
+    case (710)
+      msg = 'Quotation missing in string'
 
 !-------------------------------------------------------------------------------------------------
 ! errors related to the grid solver
diff --git a/src/YAML_parse.f90 b/src/YAML_parse.f90
index f876fd71f..4043fb6eb 100644
--- a/src/YAML_parse.f90
+++ b/src/YAML_parse.f90
@@ -72,7 +72,7 @@ recursive function parse_flow(YAML_flow) result(node)
       d = s + scan(flow_string(s+1:),':')
       e = d + find_end(flow_string(d+1:),'}')
       key = trim(adjustl(flow_string(s+1:d-1)))
-      if (key(1:1) == '"' .or. key(1:1) == "'" ) key = key(2:len(key)-1)
+      if(quotedString(key)) key = key(2:len(key)-1)
       myVal => parse_flow(flow_string(d+1:e-1))                                                     ! parse items (recursively)
 
       select type (node)
@@ -97,7 +97,7 @@ recursive function parse_flow(YAML_flow) result(node)
     allocate(tScalar::node)
       select type (node)
         class is (tScalar)
-          if (flow_string(1:1) == '"' .or. flow_string(1:1) == "'") then
+          if(quotedString(flow_string)) then
             node = trim(adjustl(flow_string(2:len(flow_string)-1)))
           else
             node = trim(adjustl(flow_string))
@@ -138,6 +138,23 @@ integer function find_end(str,e_char)
 end function find_end
 
 
+!--------------------------------------------------------------------------------------------------
+! @brief check whether a string is enclosed with single or double quotes
+!--------------------------------------------------------------------------------------------------
+logical function quotedString(line)
+
+  character(len=*), intent(in) :: line
+
+  quotedString = .false.
+
+  if (line(1:1) == '"' .or. line(1:1) == "'") then
+    quotedString = .true.
+    if(line(len(line):len(line)) /= line(1:1)) call IO_error(710,ext_msg=line)
+  endif
+
+end function quotedString
+
+
 !--------------------------------------------------------------------------------------------------
 ! @brief Returns Indentation.
 ! @details It determines the indentation level for a given block/line.
@@ -760,6 +777,8 @@ subroutine selfTest
   if (indentDepth('a')  /= 0)     error stop 'indentDepth'
   if (indentDepth('x ') /= 0)     error stop 'indentDepth'
 
+  if (.not. quotedString("'a'"))  error stop 'quotedString'
+
   if (      isFlow(' a'))         error stop 'isFLow'
   if (.not. isFlow('{'))          error stop 'isFlow'
   if (.not. isFlow(' ['))         error stop 'isFlow'